And to answer your rhetorical question: Heeeeeeeeeeellllllll fucking no, you can't choose who you fall in love with.
I agree in part with Zuri's answer, but in a way that's not changing the truth. That's denying the truth until it quietly goes away, but it doesn't entirely change it. And yes, it DOES suck. I'm sorry she has had to do it, because I have as well - and I know how badly it hurts. And it hurts like that because you're lying to yourself, and it's probably the biggest lie you'll (hopefully) ever have to to shove down your own throat.
This next paragraph I typed and realized it sounded kind of harsh, but I didn't mean for it to. I just can't think of another way to put it. I'm not in one of my more tactful modes, but don't take it the wrong way. :)
I don't care if you caught him peddling his ass for smack, don't eeeeeever deny that falling in love with Nick was anything other than perfect. Because I've sat here for months and read about how much you love him, and that kind of happiness is never free. And if I'm wrong, show me the line. I'll stand in the back of it and wait my turn.
The more you care for someone, the more you idealize them, the more of your heart you give to them, the easier it's going to be for them to fuck up. It's trying to walk on broken glass, being in a relationship. You tread softly and spread the load out, it won't hurt. You put too much pressure on just the wrong spot, and everything goes real south - real fast.
But even if everything goes to hell (which, again, from your more recent post it seems that it hasn't - and this is excellent) then you can't forget the good times.
Just last night I caught myself thinking fondly on some good moments with my ex, and I've told you (I think) everything about her. The way she treated me after those first few months, and the way things finally ended. But those first few months (followed by eight months of being screamed at for no reason, having the shit kicked out of me, and ultimately being walked out on and promptly cheated on) were perfect. And from time to time I still let myself think back to snuggling up, nuzzling, holding hands.
Things went to shit, yeah. They were shit for over twice as long as they were nice. That's a mistake I was able to learn from. That will NEVER happen again. (The relationship ends when you knee me in the balls for trying to hug you.) But those good times we had? Nothing can tarnish them. And no monumental fuck-up can take all that love away from you, Chels.
I hope you and Nick are together and happy forever, if that's what's right for you. If you always love him, I hope (and strongly believe) that he will always love you and do his best. And if it's ever not right for you two to be together, I hope you'll never forget those good times. I hope you won't let whatever stupid shit he may do keep you from thinking back and smiling about that hour and a half foot rub until you fell asleep. This man LOVES you. Nothing says that ever has to change, but even if for some ungodly reason it should? Learn, move on, grow, and remember.