Need some advice. He cheated

So me and my boyfriend had been together for about 3 months when one day he came out and told me he hung out with another girl (which was his ex) with intention of using her for sex. He told me that as soon as he got to her house all he could think about was me and how he didn’t wanna hurt me and he didn’t want things to end between us so he left and told me about it right afterwards. instead of asking the girl if they had sex or did anything, i just took his word for it. i forgave him and we went on with our relationship. he even blocked the girls number, social media, etc. and made sure i knew he didn’t care about her. about 2 months later we’re doing fine and he’s being loyal to me but i find out from a friend of his ex that they did have sex. So basically my boyfriend lied to me and told me that nothing happened when in reality he had sex with his ex girlfriend while we were still together. He told me he lied because he knew i would leave him and it was a one time thing and “the biggest mistake he’s ever made in his life” but still, why shouldn’t i be mad???!! So i tell him that i’m done with him. But he proceeded to tell me that he loves me and he doesn’t want anyone else and it was a stupid mistake and he didn’t care about the girl and he even blocked every single girl he has on his phone, social media, etc. to prove to me he didn’t want any other girl. i just don’t know what im supposed to do about this because i have so much resentment towards him about what happened. we are back together now but i don’t know if im doing what’s right. Every time that im with him all i can think about is how he had his hands intimately on another woman while we were together and it hurts me. any advice please ladies? 😔💓or if you’ve ever been cheated on, do they change? i just wanna know if this ever gets better or if i’m wasting my time.