Welcome to the forums :) Thank you for sharing your story, I know it's not always easy to do that.
First of all, I'm really sorry to hear that you made that discovery about your partner. I cannot stress enough that his actions have nothing to do with you, and they aren't a reflection of you or who you are, your self worth, your quality as a partner. He has proven himself to be untrustworthy by betraying your trust like that. I'm curious if you guys have ever spoken about it.. Does he know that you know, and how did he react?
I can't tell you the right/wrong thing to do.. But I really hope that you realise that this isn't your fault. When you're in love with someone, it's very easy to be forgiving and see the best in them, and it isn't until you're away from them that you realise who they truly are. You're able to think more logically when you aren't in the relationship or you're on a break with them.
I have very low self esteem. I was in a long term relationship with someone who showed a lot of red flags, and I made a lot of excuses for them, because I wanted to live a life with them, I loved them, I believed that they were genuinely sorry, and that they really loved me. After separating I realise how blind I was, and how I let myself be manipulated. I let him get away with things and turned a blind eye, and he continued to do things behind my back.
I hope that you can recognise this as a massive red flag to his commitment to you.. I know having low self esteem means that you let people get away with things, because at the end of the day it's a sense of love and validation or security that you may be looking for. This behaviour isn't okay, and I feel heartbroken for you that you're blaming yourself and believe it's a reflection of you, because I've been there myself. I can say that I am much better off without the other person, and they will later regret what they lost.
I don't know the entire situation. But I hope I can be the voice for you that says that you deserve better, and you have the right to be furious, hurt, and angry at your partner. He put you in the position where you have massive anxiety about the relationship, and no one should ever put you in that position.
I hope I hear from you.