I know I’m an idiot, but why is leaving so hard???

I met my now fiance on a dating app a little over a year ago. We really hit things off and would spend every weekend together and talk/text every day. As time went on I started to notice more and more random numbers texting/ late night calls etc. I eventually looked through his phone and found the unthinkable. I wanted to leave, but stayed since at that point we weren’t “officially in a relationship”. We made things official, he vowed to stop and cut everything off and things still continued.

After months and months, though he is no longer having intercourse with other people it is still one thing or another. looking through his phone throughout the multiple months it’s always an app, talking to someone, deleting messages etc etc. I always say I want to leave but get sucked back in to the promises of change etc. more recently, we have come to the conclusion that he is a sex addict. He wants to speak help and therapy for it and to save our relationship but at this point I’m so broken.

I’ve cried myself to sleep multiple times when I find things. Thinking about what I’ve had to endure and how can this person possibly love me if they do this to me??. I feel weak for not leaving. I love him so much and keep trying to make it work but at what cost? Why can’t I leave?? Will he ever actually change if he seeks help for his problem?

I wish I could get the balls to just walk up and leave and not look back. Every time I try, I fail.