Karma of Kissing

I’m struggling with what karma is and what is not… I kissed another girl while I was in a relationship, for what I’m really sorry and I have many troubles to forgive myself what I did. I’m trying my best everyday to fix it. In my mind there is always a thought if the loved one will do the same to me. I read many books and articles, but karma is so complicated to understand. Maybe it’s a silly question, but please let me know what do you think, if I cheated it means that it will happen back to me? I can’t go back in time and fix it but I realized my mistake and don’t know how to deal with it. I have problems to forgive myself and I just hope everything will work out with my (ex)girlfriend and that one day we will be back together.