Bf and friend both lied to me

So basically, before my boyfriend and I were official, we “ talked” (otherwise known as dating without an official title) for like 6 months. For most of those 6 months he didn’t know if he wanted to be in a relationship with me so I eventually got tired of waiting and said I was done. A week later I started goin out on dates with someone else (very casual) and I let him know bc he was trying to get back together with me. He would text and try to call me every day. He would tell me how much I mean to him and how he was ready to settle down. He told me he loved me that week too.

But there’s something he didn’t tell me. He didn’t tell me that him and one of my friends slept together. Granted, her and I werent super close at the time, but we WERE friends. Her and I ended up getting close after that though, ironically, because i didn’t know about what had happened. This happened in nov 2017 and it’s now April 2019 and I barely found out last week via text by him. My trust is so broken because they both lied to me. And they both kept it a secret from me. My trust is so shattered. I don’t know what to do. Please give me advice.