Seems like you're the one who can't handle it, not him. You have developed stronger feelings for him than he has for you. That happens every day.

Be straight with him. Tell him that it bothers him that he has this other relationship, that you want him to have a one-on-one relationship with you (don't pretend you don't), and that's why you can't be friends with him.

He didn't change - you did. And that's fine. But blaming him is not right. He was up front with you from the beginning.

That's the apology you need to make - not for ending your friendship, which it seems like it is a very healthy thing for you to do - but for suddenly judging him so terribly, when it's obviously a cover to keep yourself from being vulnerable and admitting how you really feel about him.