Relationship what do u think?

So recently i checked my boyfriend’s phone. We’ve been together for a year now. I have never done it before because to me privacy is very important. Yet i had this very bad feeling...Unfortunatelly i found out that he kissed a girl a few months earlier (while we were together). Before he met me there was something between them, he even wanted to be with her but she turned him down.

When we started dating she got jealous and told him that she was in love with him but he said that he is No longer interested. Few months later me and my boyfriend were at a party and we started arguing and i left. The next day (when i was at home) he was still drinking after that party and he was again very drunk and this girl came over to see him and his friends. That night they were kissing and sometimes after that situation they met a couple times. He told her that he was drunk and he should not do that.

I know being drunk is not an excuse. I really love him. We live together. I was planning my future with him but i cant stand a thought that while being together he kissed someone else...I asked him to block her everywhere and he did it. They have No contact. But i am scared it may happen again. Please have you got any suggestions? Has it happen to you?