AITA for starting a relationship with my best friend’s ex?

So here is the story. When I was 17 I befriended a girl in college. Her name is Lisa. After few weeks she began dating that guy named Ben. They were in love and their relationship was the first serious thing for both of them. Lisa quickly became my best friend, we shared all of our secrets, drama, sad things etc. with each other. She basically became my sister. Even so, we started hanging out with our families present. Lisa’s and Ben’s relationship steadily grew and Ben became my best friend too. There wasn’t a thing in my life I couldn’t tell them both. Over a few years Ben became my brother. The thing is, my best friends relationship came apart after 5,5 years. Ben proposed and after 5 month since the proposal, Lisa cheated on him (Ben didn’t know, but I did, as she was still my best friend). I consoled them both after the terrible break up, listened to both sides, but didn’t side with anyone. At the time we were 23-24, so basically grown adults. After the break up Lisa started dating the man she cheated with. They moved in together and the guy proposed. By that time it was a year and a half since Ben and Lisa broke up. And as for me, after 7 years of friendship I realised I really loved Ben, more than a friend or a brother. Nothing ever happened between us before, not even a hint of romance, as he was in love with Lisa and I had my previous boyfriends. I thought maybe it was worth a shot: Lisa is preparing to get married to the man of her dreams, and me and Ben have realised that we could work as a couple. I was nervous telling how I felt to Ben, as I was terrified of Lisa’s reaction when she would find out. He reassured me, that everything is going to be great and we started dating. He’s been my best friend for years and the relationship with him is very smooth. So, when Lisa found out about me dating her ex after a year and a half of them breaking up, she was livid. She told me that I was a terrible friend, that I betrayed her and I should have thought about our friendship before getting with Ben. I feel guilty, as I get that they’ve been together for such a long period of time, but I guess a lot of time has passed. We don’t even live in the same city as Lisa anymore, we don’t have any mutual friends. Guys, AITA? Should I not had gotten with my best friend’s ex?