I cheated on my girlfriend a year ago. Now we're talking again.

Alright so almost a year ago I cheated on my girlfriend, Ashley. I ended up breaking up with her after I did that because I thought the other girl was what I wanted but I was wrong. Me and the other girl didn't last long. Ashley took this very hard because we had a good, serious relationship. We didn't talk over the summer but when school started this year seeing her made me miss her a lot. I tried talking to her a few times and told her how I bad I felt and how much I wish I hadn't done that to her. She told me that she forgave me a long time ago but made a promise to herself and her friends that she would never take me back. We tried being just friends for awhile until she got a boyfriend (the first since me) and then I left them alone because she seemed happy. Their relationship didn't last more than a few weeks and she messaged me first this time. We even had a long phone conversation like we used to. I brought up the topic of me and her again and she now said that she now knew she needed more time without a relationship. This was a few months ago and now recently we've been talking more than usual and one of my friends brought up to her if she wanted to go to prom with me. She told him that she actually wants to but her dad probably would not like it. He also found out that she was under the impression I had been with a lot of people over the summer and he told her those were just rumors which I think she was relived about.

So now that you know the story I guess my question is what is your advice on what I should say to her or even if I should do anything about her dad. Or even if there's something I could do to really make her happy. I really think her and I were meant to be together.