My ex's ex & my secret

I have an ex from about 8/9 years ago, who just keeps popping up every now and again. His ex (who he now shares a child with) hates me. Early this year, me and my ex started speaking and his ex found out. She messaged me saying that I was 'in for a big shock soon' and that she had proof that I was having an affair with my ex (even though I wasn't). I didn't see this message until September. My ex came to the city I was living in with his friend and I met them without telling my boyfriend. I was a bit messed up at the time - problems with work, my relationship and I was also pregnant and didn't know yet! I got extremely drunk and let my ex kiss me. His ex doesn't know about this and if she did, she would stop him from seeing his child. My boyfriend doesn't know and I can't tell him because he will leave me and I am not prepared to lose the love of my life over a drunken, overly hormonal, mistake. I have no feelings for my ex at all, but I don't know how I can keep a secret, especially if his ex finds out .... I need to forget about it and move on. I love and adore my boyfriend and I need to rid this guilt. Any help would be overly appreciated! Thanks x