I've done the Christians-having-sex thing in the past so I know pretty accurately what's going though your mind here. Firstly be aware that if you have repented, you are forgiven and washed clean; don't let the enemy tell you you've blown it irrevocably because that's not true. I'm presuming pregnancy and disease hasn't been a result so thank God for that.
I think you need to talk to your ex honestly and see what she wants. If she wants it to stay ended then you have to accept that. 20 is young to make a serious commitment but not necessarily impossible; it might be wise though to wait until she's graduated before making things too serious. If you do get back together, make sure God is at the centre of your relationship, commit it all to him, and be wise about not being alone together; I know from personal experience that one sexual encounter with someone can make it hard to resist temptation next time, as if it's a slippery slope.
Have you asked God what He wants for you both regarding this? What has He said?
I'll be praying for you.
Jules