I messed up and dont know what to do need advice

So i dated this guy that we fell deeply in love over summer, he had to move away to a different state and we tried the long distance for 3 months but it started getting too hard for him. So we ended it until i get news if ill be able to move closer to him or not. We periodically keep in touch but we try not to cause it makes things harder. And while we have been broken up, his cousin which is or was at least my friend came over to the city i live in and we hung out like we used to at a small getty in their house. Several drinks later one thing led to another and i ended up making out with his cousin. I feel like shit and so does his cousin and for now we said we would pretend like nothing happened and leave it at that. But i feel really guilty and if i wind up getting the news im moving closer and we try to start up again i feel like i should tell him. But i am so scared of telling him.. help any idea what to do im soo lost... plus he is coming to visit his family in my city next month and said he would like to see me but we wouldnt be getting back together what should i do?