Confused

So my ex and I broke up and it’s been almost 7months since we spoke.. and today he popped up out the blue... I missed him so much. But I told myself once I have sex with someone else I will never be able to deal with him again (out of feeling guilty). New Year’s <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>/day I got drunk ... and had sex with someone I was talking to. Something I wanted to do but something I would never do sober.. it was always just a thought while being sober. But of course everybody is so careless when they’re drunk.. and it happened sooooo fast... I feel so guilty . Even before he popped up.. because honestly I’m still in love with him.. he popping up tonight made me feel worse!! We’ve been together for 7yrs.. that was my best friend ... and i know him will ask if i had sex with someone else but should I tell him or keep it to myself since we weren’t together when it was done? (we’re still not together)

We broke up after our son passed it was hard on both of us.