Spoonfeed me please.

I'll just copy/paste an earlier post of mine, where I try to convey my interpretation of the movie. This is subjective, by the way, but I hope it helps:

So, we know at the end of the movie that Trevor killed Nicholas with his car and then fled the scene to avoid prison. He's then seen visiting many different places in the company of Maria, and even Nicholas, who couldn't possibly be there.
Someone saw the presence of Nicholas in Trevor's life as a sign that Trevor is actually dead. Some people think the airport is Heaven, or Hell, or Purgatory (which is, in a way, the most fitting of the three). That is interesting, but not entirely fitting to my interpretation of the story. I would like to start a short analysis of this beautiful film, starting exactly from that place, which is one of the core points of Trevor's year-long-trip into guilt.

So, the airport. The airport is really an airport, physically speaking. It happens to be on the way to DOWNTOWN", where Trevor (whom I believe to be still alive and breathing) knows he has to go but never finds the courage to go, because he could not admit to his crime, not even to himself.
Any time he would get to the crossroads, he would make a left to the Airport when he should have made a right to Downtown. Once at the airport, he would sit at the bar, and fantasize about getting to know Maria, befriending her, making it up to her by leaving her generous tips, making plans with her and her son for Mother's day (the day he ruined her life), etc... when in reality Maria wasn't even there.
Notice how, every time he watches the clock getting stuck at 1.30 (the time on his car's dashboard when he killed Nicholas), he gets weird for a second, then looks at Maria (whom, I repeat, isn't there) and smiles, then goes on being nice and friendly. He clearly does not want anything to remind him of what he's done, and wants to live this delusion of being nice to Maria and Nicholas forever. In doing so, he's leading himself straight to hell, as pointed out by the filmmakers during the Route 666 ride he fantasizes going on with Nicholas. The crossroads, in the ride, between Hell (the airport) and Salvation (confessing the crime downtown) is the same one he's been at every single night in the past year, and every time he chooses to go left (airport/hell, not admitting to his crime, deluding himself that everything can be just fine).

Ivan, on the other hand, he's just the part of him who is fully aware of Trevor's crime. He's Trevor himself, Trevor trying to remind Trevor that he's a killer and needs to be punished for his crime. He's the one who's been leaving all those notes. Meaning, Trevor has.
Even when it comes to the notes, Trevor wants to live in a delusional reality where everything is fine with him, but everyone's against him and want to set him up. Notice, when he fills in the spaces on the hangman post-it, he deliberately chooses a pencil, when a black marker was readily available, and even goes as far as to deliberately change his own handwriting, which we know is the one on the post-its, because he himself compares it with the one on the Mother's Day card he gave Maria in his other delusion. When Ivan finally tells him that he's the one who's been writing those notes, he goes to the fridge, picks up the note, deliberately chooses the black marker, and fills in the missing K in his own, unaltered handwriting.

His co-workers are also important to Trevor's personal trouble, and their gradually becoming more and more hostile towards him (which Trevor does nothing to prevent or stop, by the way, and there's a reason why) make Trevor angrier, more delusional and more paranoid as the days go by. But could he be feeding this? Could he be willingly feeding his coworkers' gradual disliking and suspicion towards him? I think so.
Think about it: once we know that he knows exactly what he's done, and just doesn't want to remember/admit it, it becomes pretty clear that he's willingly forcing himself into total alienation from people. Being alone, he believes, will make help him come to terms with himself. He knows exactly who Ivan is (he says so to himself in the mirror), and he knows that he will have to face him (his conscience) very soon, because he's been trying to hide his crime for about as long as he possibly could, and now he can't take it anymore. He knows he's going away. But can he say goodbye to his whole life? He can't do that.
The reason why he takes his guilt out on his co-workers is because they're the people in his life who used to be friends, and now aren't feeling comfortable any longer when he's around. In his delusion, he wants to give himself an excuse to dislike and hate them, not the other way around, because the fact that his colleagues, who used to like him, do not like him anymore, just remind him of what a wreck his life has become, and what a bad person he really is. The most clear example of this is Reynolds.
Reynolds was, among Trevor's co-workers, the one he was the closest to. They were friends at work, and outside too. Going fishing and such.
As Trevor is shown in a picture with Reynolds from one of their fishing trips as being healthy and well-fed, and since the fish they show to have caught in the picture is revealed to be still in Trevor's freezer, we might assume that that fishing trip with Reynolds was Trevor's last day as a normal human being, before he ran over Nicholas. It may have even happened on the same day. That could be the reason why Trevor tries so hard to convince himself that he is not in the picture; me could assume that remembering that fishing trip reminds him of what he did on that day.

Trevor even sets himself up with Stevie, so that he could blame her of something, anything, start a stupid fight and badly break up their friendship, before he turned himself in. Stevie had been there for him during a whole year, counseling him almost like a psychoanalyst. She'd been nothing but good to him, and yet he could never confess his crime even to her, who would have most likely helped him and encouraged him to do the right thing: go downtown. In the end, he had to blame even her for something, and detach himself from the only person who cared for him, reach total isolation, before he could come to terms with his own guilt.
Trevor's mother ultimately fits in the story as a reminder of the fact that Trevor knows what it's like to lose someone we love, he knows what the love of a mother for her child is, and he knows what he has destroyed with his actions, the entity of his crime. The picture of him with his mother at the amusement park helped him for a moment to build in his head his own cathartic "Mother's Day with Nick and Maria", which ultimately went horribly wrong when the hellish ride with Nicholas, and the realization that his own handwriting on the card he gave Maria is the same as on the post-its, only further kicked reality in his face. He's living a delusion. Nothing is gonna be alright. He has to pay for his crime.

There are many theories that are much more complex than needed.

Trevor's lack of sleep has him hallucinating, and his guilt manifests itself in many ways.

There was no theme park visit in present day. He went there when he was a kid during happy days. He imagines the boy's mother, the boy that he hit, perhaps visiting that park too, just like he did with his mom. But when he imagines himself there along with them, his guilt manifests itself in the Route 666 ride. He is feeling unimaginably guilty because his actions robbed that mom and child from having the happy childhood he did.

Trevor has Deja Vu because the real waitress, a completely different person, told him something that he later imagines Maria saying. There is no Maria.

Here is the reality. Trevor runs down the kid and flees. He tries to forget what he did, but his guilt gnaws at him. He gets rid of the car. He cannot sleep, he runs his body down. He loses tons of weight. He takes up with a prostitute. He hangs out an an airport diner. His mind invents an alter-ego, the part of him who was free-wheeling, carefree, Firebird-driving jerk, and he sees that side of him now as an imaginary person. He so wishes that the boy was alive and that the mom who loved him were a family again, he invents them and inserts himself into their lives, to assuage his guilt. But reality keeps seeping in.

He was friends with Reynolds, it was he who went fishing with him--the fish that now rot in his freezer because he did not pay his bill-- but he only sees his alter ego in the picture. Outside of his dream state, in his normal state, his illusions appear to be a conspiracy against him.

The hangman...Mother, no Miller, no KIller, was made by him, his subconscious desire to come to terms with what he is, a killer.

When he comes to terms with who he is, he can finally sleep.

Fantastic movie.