Don't talk to her. She's a stranger now. You don't owe her anything and she doesn't owe you anything. You'll still see her but completely cut her out of your life in every possible way, excluding the time you see her in class.

I'm assuming you don't have many hobbies, friends or a job. Try to get all of those asap. You're going to waste the best/young years of your life thinking about some girl who isn't even worth it.

Do/read/research/practice everything you possibly can do to build up self confidence. You lack self confidence/self esteem and this is primarily your issue, I believe.

Stop focusing on the past and live in the present. I've very recently made this mistake with my ex-girlfriend and sometimes still do to be honest but it's a huge waste of time.

Don't resent your friend, she's your ex, it doesn't matter. (Shitty thing to do, but it's college and you can't blame him if he sucks at getting girls and finally got one.) Don't resent your ex, either, for that matter.