In the climactic fight scene between Selene and Marius, I want to know why Selene needed to bite herself to remember stuff and also how she could suddenly be strong enough to rip Marius' backbone out with her bare hands. It's the climax of the fight scene and such an ending can seem like Deus ex Machina without some proper explanation.
As for your first question: I don't think blood memories are the same as 'normal' memories. Blood memories are more like enduced visions, stronger than normal memories. And she does it to remember Michael and draw strength from her love for him, to fight Marius and avenge Michael. She had just learned that Michael has been killed and was about to give up. That's how I interpreted it.
Why would she need to remember Michael by biting herself? I get that memories fade over time but it's not like it's been a hundred years since she last saw him. Unless someone erased her memories this makes no sense.
Visions enduced by blood drinking VS internal memory => Compare it to rewatching an emotionally touching movie you've seen to just remembering the movie. Or seeing someone you're fond of in front of you VS just remembering them. The first will be a lot more engaging than the latter.
Why is that needed for motivation though? What, seeing Michael being killed wasn't enough motivation for her to get revenge on? Unless her feelings for Michael had changed and she needed to remind herself of what she used to feel for him, then this makes no sense. Because if her feelings were still the same just seeing him being drained to death should be enough. Seeing him killed sure was enough motivation during Evolution.
True. Why should I want a reasonable explanation as to why it was necessary for Selene to do that? Oh right so the plot can make sense. Eh who cares about plot.