He cheated

My boyfriend and I got back together 4 months ago after going through a lot of infidelity for 2 years. I thought things were good... but I never really trusted him after all we’d been through. We were arguing a lot because of trust and I guess three weeks ago he slept with his ex. He claims it was because he was insecure about our relationship but I just don’t understand. He says he’s in love with me and I’m everything to him but how if he could so willingly cheat on me. AND we had sex after wards. I didn’t find out from him either, I found out from HER. He begged me for another chance and now he wants to start over, but i just can’t stop thinking about them having sex. I need advice..