I’m in love with my bestfriend

Ok so i’ve been really good friends with this guy for over 3 years and we have a great relationships. Back in june we hooked up and i friendzoned him right after. The thing is that I later learned that he had feelings for me at the time we hooked up (in june). We didn’t speak all summer because it was just super awkward. But now we started talking again and things seem to be getting back to normal. BUT MAJOR PROBLEM ... I just realized that I have and have had feelings for him for a long time and I had been ignoring them. Anyways I want to tell him but he has a girlfriend and I don’t want to ruin that for him. What should I do??