(Cliche/pathetic I know but) I'm in love with him but he's with her. Help?

I've been best friends with this guy for about 3 years and we've been through a lot together. He's a veeery attractive guy and the type lots of girls like, and to be honest I have no clue why he's even friends with a fat, ugly nobody girl like me.
Sorry, anyway, about 6 months ago he and my other best friend (a girl just to clarify haha) got into a relationship together. You've already guessed from the title of this question that I have very strong feelings for this guy, actually, to be honest I've been in love with him for a long time but he's never known how I've felt. I know that he'd never go out with a girl like me and the girl he's with now seems to be perfect for him (thin, blonde, really pretty etc).
It kills me to see them together every day and I can't take it much longer. I want him to be happy, I want them both to be happy, even if I'm miserable, but I'm worried he's started to notice my depression. I suck at hiding things from him, but if he finds out I'm terrified our friendship will be over! I can't lose him, he means so much to me.

Sorry if this was kind of long and I know it sounds really pathetic but I don't know what to do. This guy saved my life in more ways than one, he makes me feel so happy that I forget about everything else, even the things that make me depressed. He's been there for me when no one else has. Please help.

Awww I want to cry now... *takes box of Kleenex*

Ok I'm good.

Hunny bunny, first of all, just because a girl is blonde and skinny doesn't mean she's pretty to everyone. I'm a proud brunette!!

Next, it sounds to me like you and this dude have a "best friend" relationship. I mean, I know you don't want it that way, but we're just stating facts for now, kay?

OK SO... you're besties. Which (hopefully) means:

1. You can tell him anything
2. He won't judge you
3. He will try to make you feel better or more understanding

Seriously, you've known this guy for 3 years! And you said so yourself that he'll know all your feelings. I think you should just sit down and talk with him. I know it sounds hard, but again, I'm assuming you're best friends, especially if he's been there for you, "when no one else has." :)