Advice needed

I have been with my partner for over 3 years now and we have had a very healthy and happy relationship. Just a few weeks ago I found that he had sent explicit texts and images to another woman. I was very hurt and had an argument with him. After alit of conversations and thinking I decided that I would not leave him and that we would work on our relationship together after I made sure that this was a one time occurrence and he seemed very regretful and sorry. Just a few nights ago I had far too much to drink and very briefly ended up kissing another man, I told my partner straight away. This was the biggest mistake I have ever made in my whole life and I cannot put in to words how much I regret the horrible decision I made. I couldn't believe I would do such a thing and hurt my partner in such a way. My partner now doesn't think he can give me a chance and cannot see himself forgiving me. I'm at a loss for what to do and any advice would be appreciated