Hey dude. I'm same age as you, and I too have suffered through heartbreak and awkward relationships in the past. I know the feeling of how everything piles up. I'm not gonna spam your thread with my own history, but I just want to make it clear that I totally sympathize with your situation.

First of all, if you can, try to let the whole "I want to fuck Mark" thing slide. Obviously this will be really hard for you, seeing as how you seem to be head over heels for this girl, and she's telling you she wants your best friend's body. The reason you've got to let it go is that you don't know what's really motivating her to say that, and therefore acting on the situation would almost definitely cause more problems.

You don't need to tell her how you feel about her right now. As a matter of fact, I'd advise AGAINST expressing your love. Even if she feels the same way, it could scare her a bit. Just ask her out. Don't overthink the matter. If she's going out of her way to talk to you and smacking your ass everyday, there is a 100% guarantee she'll say "yes" if you ask her to a movie, or something. So take her out, and have a good time.

Then, take her out again, and kiss her. It took me 4 years of overthinking everything to realize that just kissing someone is much better than telling them how you feel. It's like the difference between showing someone a movie or describing it to them. If you find a "moment" on the date, then go for it. Otherwise, waiting for the doorstep or a similar place is a good idea, but do it.

You're not cursed, my friend. You just have stronger emotions than most people. This may seem like a curse, but eventually you'll learn how strong your emotions can make you.

I hope this helps... I've been skimming back and forth to your post to try to cover everything. If you want to talk anything else out just let me know.

Peace.