having sex after a break up

So my ex and i got into a fight but had already broken up a few days before. he ended up blocking me so i didn’t think we would talk again. i had meaningless sex that night. my ex ended up texting me again and saying he wants to try us again. i couldn’t lie to him and so he knows i had sex with someone else. he is the boy who took my virginity so it’s kind of a big deal to us both. do any of you know anything i can say or do to make this situation better?