It’s kinda complicated. There is a boy that I really, really, really, REALLY like. It’s like everything he does is what I’ve always wanted in a Significant Other. He’s really nice, he’s funny, he’s cute, etc, etc…. *sigh* Anyway, the problem is that he really, really, really, REALLY likes another girl. And she happens to be one of my closest friends. (Did I mention that she’s REALLY pretty and amazing? Anyway, she is.) Now this girl doesn’t often return the affection shown her but she likes this guy and what they have between them is amazing. I made up my mind long ago that I’m not the sort of person to come between any couple, but every time I see them together or I’m around them, he always does something that makes me like him even more. And she always replies in the same way I would have because we have similar personalities. This makes it hard to go all Taylor Swift "You Belong With Me" on her.
Jono, I don’t know what to do. I really want to stay friends with both of them because I’ll admit it, they’re pretty cool. But when I hang out with any of them, my heart shrivels up and dies a little more each time. Because of them, I can’t even hang out with my regular group anymore.
It's sort of hard for me to parse the nature of this love triangle, Sparkler, or love... right angle... or whatever the shape of this situation is. You really like him, he really likes her, and she likes him, but not as realily? Or she does, but she just isn't as keen about returning his affections? I'm picturing this guy fawning over her and lovingly telling her she's pretty, and her being like "No, you're pretty. Pretty stupid! Ha ha," while in the background you tear your hear out in throes of ennui.
You say you want to let them be together, and that you long ago made up your mind never to come between any couple. But I'm pretty sure that when I was in high school, I made up my mind to be a freelance emo poet who lived in the woods; it's a little early for deciding what rigid personal code you're bound by. I'm not twirling my moustache and wringing my hands and advising you to break them up because of how much I love mischief. I'm just saying that you're not exactly making this sound like they're head-over-heels in love.
If you (commendably) don't want to pursue this guy with the situation as it is, then there are really only two things that can happen: you remove yourself from the situation or you don't. Here, I would imagine, is how those options would play out.
Option 1: Live with it
You resolve to be the bigger person, and to endure their relationship without exploding in a fit of self-pity. You prepare to deal with the most grueling possible situations, like if you somehow wind up seated between the two of them at the movies, and he has to lean over you to tell her stuff like "Oh can you pass the sexy? Because you're hogging it all," and they basically have a baby right there in front of you. You soldier on through all of this, constantly hoping that she gets hit by a convenient meteorite. Without actually intending to mack on your friend's boy, you find yourself doing all kinds of vaguely flirty things without realizing it, like if you talk to him on the phone, you'll say you just came out of the shower, when in reality you have been eating bacon in your sweatpants. Why are you doing this? Are you unconsciously trying to win him over? Probably.
Option 2: Friendship hiatus
You avoid any situation where you will be alone with the two of them, sticking to other friend groups where you won't be confronted by their smoochy affections. You make up nonsensical and occasionally hostile excuses every time one of them asks you to do something. ("Sorry, I can't go for ice cream, it turns out I'm allergic to your horrible face.") It's certainly easier not to have to watch them together, but you also start to drift apart from them. You channel your energy into convincing yourself you have a crush on some other, runner-up guy, whose name is Blandtonio. He is okay, sort of. Months and months down the line, you realize you're not as bothered by the situation anymore.
I wish I could tell you which of these was the best thing to do, but I can't, because I'm not a liar or wizard. I don't know if this is unbearable for you or merely awful. But I can tell you two things that are definitely true: everybody gets over everybody eventually, and staying friends with people is almost always smarter than not doing that. I know this isn't fun advice to hear, and I wish I could just be like "punch your BFF in the nose and steal her man," but they would revoke my advice license if I did that. All I can tell you is that most things get very slightly easier to handle every passing day, and that having two close friends in the long-term is much better than the alternative.