Hey you are not obligated to stay in contact with her, especially after what you went through with her.  

Rekindling the connection you had with her will most likely bring you back to those days and make you feel even more bad.

You have to let it go and move on, in my opinion.

The only way you can separate sex with from hurting someone else is to understand what the other person you're in a relationship truly wants. Have an open conversation with your significant other or partner on what you both want, so you can reach a common ground and get to a level of comfortability whereby you don't feel as if you're hurting them.