My ex came back?

My ex and I were together for 2 years we broke up and a week later he was in a new relationship this was 5 months ago. Well his gf and him broke up so he found me on fb. He sent me a friend request which I ended up accepting and then he messaged me and apologized. I'm in a relationship now and have been for the past 3 months and I told my boyfriend he was okay with it at first cause I showed him the convo. But then today I forgot I was logged into my Facebook on my boyfriends phone and he saw that my ex messaged me again and he got mad and asked me to block my ex. Which out of respect for my boyfriend I did. But I feel bad because I didn't tell my ex I was or anything and we were actually talking instead of fighting like we always do. Now I'm confused. Should I unblock my ex and tell him the reason I'm blocking him or should I leave it alone. My boyfriend knows how I am when it comes to this guy, I have always gone back to him and I know he's afraid that I'll do it this time. I mean I still love my ex and always will but I can't go through what I did for the 2 years of our relationship. WHAT DO I DO?! HELP!