Ashamed

So I left my boyfriend because he was pressuring me to have sex and it hurt a lot. I was really in love and still two months after the break up I used to cry when I saw him with other girls. So I got tires of trying to stay strong when i really wasn't and ended accepting the attention of my trainer. The problem is that he has a girlfriend which was pregnant in the moment. I was so vulnerable and didn't want to be alone that it seemed great to me the fact that I was going to have the attention of a man without being involved in anything serious. This continue for about four months until I started to realize what I was doing and started getting feelings, which was not part of the plan, so i put a stop to it. And now that I'm pretty conscious about all I this I am really ashamed of it, i cannot understand how I was able to continue that affair for 4 months. I really regret doing it. I'm grateful that she and my dad (which he knows) never knew about it and nothing bad happened. Maybe that is selfish of me but I cannot feel another way. I know I am never going back to that, but the affair was not sexually we necer had sex. it was more emotionally we just kissed and had very deep conversations. And that created a good friendship which I now miss. But I know that if I try to maintain that friendship everything is going to start all over. Because he has offered me to come back, and he disliked my decision of putting a stop to the affair.