I fucked up

I fucked up... I messaged an ex. This ex in particular was someone who at the time was hard to let go but I needed to. My current boyfriend made it alittle bit easy to say goodbye. My history with the ex is complicated. He and I had been off&on for SIX YEARS! There would be times we wouldn't talk for about a year but In between time I never dated anyone. I wanted to stay available for whenever he called. But.. on his end he had an on again-off again girlfriend. I didn't know about her till our 2nd yr. Even at that, that didn't stop us. I say us because we both agreed on still seeing each other no matter what happened. And we did so for FOUR MORE YEARS! The gf knew about me and confronted me on the phone but that still didn't stop neither one of us. We continued to see each other until it got more complicated when the gf had a baby and still managed to keep talking. I tried expressing myself to him by telling him that with the baby now, there's never going to be us; me and him. But he kept insisting that he didn't want that to not happen. So I held on. Then I meet my current boyfriend. We went out together a few times and had actually spent quality time together. For the first time in 6 yrs, I went out with someone else. Even though my ex and I hadn't talked for a few months I felt like I was cheating on him all the while he was with his gf. With the time I was spending with my bf I got to the point, I felt comfortable enough to tell him about my ex. We hadn't yet been official but he expressed his feelings and I felt the same way. I still hadn't talked my ex until he texted me one day when my bf and I were together. I told my bf and he gave me an ultimatum. So I told my ex that I could no longer do our thing and that he had needed to go our separate ways because I have someone who is now wanting to invest their full time on me.  He said ok and tried for me to go see him one last time but I didn't go and it literally broke me heart. Since then, I have been happy and love my bf. He is a great man. We have been together almost 2yrs and I have yet to find something I don't like about him. We talk about EVERYTHING. I WANT TO TELL HIM WHAT I DID BUT THEN AGAIN I DONT BECAUSE I MIGHT LOSE HIM. There wasn't much said in the message. I jus asked if the number I had was still his and it was. Idk what tempted me to messsge him but I know I fucked up by doing so. Idk what to tell my bf if I even decide to tell him.