What do i do? My friend from college and i have drifted apart?

[Aaron]

Get over it, that's all.

I hope i don't sound harsh, you're in a very awkward situation but i think it's best that you do what you feel like doing despite your ex's presence, you can't let his existence ruin your friendships or inhibit your living, casually ask your friend if she'd like to catch up sometime, keep in contact, and if you're invited somewhere, go.

If you and your ex are uncomfortable in the same room, it should be him that leaves, not you. You have your own things to do and say, and she should be your friend too, despite your ex.

Making the effort may make you feel uncomfortable but you'll learn and you'll get used to it, she's your friend too, don't become a stranger because your ex makes you uncomfortable.

Edit: also if you're invited, then doesn't that mean she would like to see you too? Your problems with your ex shouldn't become a problem with your friendships!