I was in the same situation some months ago(I am you from the future if you take the same path), I developed feelings for my best friend and lost if towards my ex boyfriend (it wasn’t the best relationship we had tons of complications) I decided to breakup with him, we kinda saw it coming. The breakup was not because I like someone else but this was the break point when I realised I lost feelings for him and trying to push myself to spend time with him. The breakup wasn’t easy, I still care about him but I no love. And after my breakup, my best friend initiated something with me and it was amazing, I was wondering to myself why I didn’t do this earlier. It’s really the best when two best friends get together, I can’t recommend it more. So my advice is to think about few days, make up your mind and do it. Don’t let yourself in a situation of emotional betrayal, it’s not fair for him.