'So this is what this feels like.'

That for the fist time in his life he didn't get what he wanted or got rejected... You know his mom made sure he always had everything he wanted so it was his first heartbreak.

Preume he meants this is what it feels like not getting what you want.

Its that man again!!

Probably what it feels like to be the one who is rejected. Since he was such a sought after catch (a John F. Kennedy Jr. type) he probably had women throwing themselves at him and he was always the one to break the relationship off. Maybe when he let her "humiliate" him, he could sympathize as he was always the dumper in the past. That's the way I took it anyway.

I think he meant he knew he really didn't want to marry her either

I always just assumed he was kind of suspecting it would happen and that Melanie would realize she's still in love with Jake and leave him. So he may have been imagining what it was like to be dumped at the altar, and when it happened...this was it.