Should i tell my ex who cheated on me that i slept with someone?

I had the perfect relationship i was with my bf for 2yrs and 6 months, all my friends and family used to tell me i was lucky and it was hard to find a guy like him, he would do anything for me and he got a long with everyone as well. Recently i found out that my bf was lying and cheating. He was flirting with girls online and told his friends that he was sleeping with other people. i asked him y he did that he said his friends were making fun of him because he was sleeping only with me so he made all that stuff up and about the flirting he said he felt like i was going to wake up one day and leave him..which i thought was stupid. so i broke up with him even though i still love him. He apologized a million times he even cried one time telling me how he cant stop thinking about me and how he regrets everything but i told him that i need sometime to think and decide if i want to get back with him. Something else happened when i was single there was this guy who been telling me that he likes me for almost 2yrs now so i hooked up with him one weekend after that he wants me to be his gf. I like this guy but there is one problem i feel like he isnt a bf material he got a lot of girls on him. He told me that he is faithful when he is in a relationship and i even heard he was with this one girl for almost 3yrs which suprised me. the real problem is now i want to get back with my ex and i don't know how to tell him that i hooked up with that guy. i have always been honest with him. i feel like if we r going to have a good relationship i shouldnt lie about this plus its going to be in the back of my mind for the rest of my life if i dont tell him now. what should i do??