I kissed a long time friend of mine right after my s/o and I broke up

So, my s/o and I broke up and I was upset so after work I went to a long time friend of mines house and we made out and nothing more. My s/o wants to get back together and I want to as well but I don't want to tell him what happened between my friend and I because his heart wouldn't be able to handle it, even if it is as small as kissing. What should I do?