Is fwb with ex a bad idea?

For context, I'm 19f and he's 18m. We were each other's first relationships and we dated for around 3 months. It couldn't work out due to compatibility issues. Initially I was quite devastated when he suggested breaking up (I had anxious attachment and am quite a lonely person in general so...) but soon enough I was at peace with it as I realized he had lost feelings and other stuff. We ended things recently on very good terms deciding to remain friends.

Within those 3 months, we did get quite physical with each other. I'm very comfortable with him, he is with me and there's physical attraction between us. Both of us always really enjoyed everything we did. I'm pretty sure I can separate the emotional side from the physical side of it. I don't see him in a romantic way anymore because the imperfections of the relationship always come to mind whenever a memory comes up.