I think that it's guilt. There's a good chance that you'll overcome those feelings but guilt can linger with you for your whole life.
I know that feel, bro.
Or, at least, thinking you do
lol as if I care.
That's only painful if they don't care at all, and even then not as painful as helplessly watching a close friend go through a terrible experience.
I feel like it also works vice versa, like when you know someone likes you more than you like them. And I don't mean that conceitedly. Like when a guy is so nice and you know he's in love with you but you just don't feel that way, it sucks.