Boy bestfriend (IK IT'S LONG BUT PLZ READ I NEED ADVICE)

Me and my boy bestfriend are really close. To the point where ppl thought that we were together. I have feelings for him but they were on and off for the past year. But this time I have had really strong feelings for the past 5 mths and that's the longest I have like him for bc usually I did get feelings for him again but it wld go away in 2 weeks time. The problem is he has a gf. But I knew that he liked me bc it shows and my friends and even my older cousin told me that it's obvious that he likes me. One night while I was talking to him and my cousin said out loud just tell her u like her already! And without hesitation He began to confess that He does like me. But he said doesn't want to break up with her bc his mom will be angry with him and ground him and he doesn't want to cheat on his gf. But now idk what to do bc I see him and his gf are getting closer and he's always talking to her and it feels like we're fading and I can't do anything abt it bc I don't want to seem jealous or anything. And they've been tgt for like 9/10 mths now and just now is gonna be a year. Idk what to do.I don't want to be a hoe n mash up their relationship but at the same time he's all I really want. I just want my feelings for him to go away so I can leave them in their relationship but that's difficult bc every time I see him or tlk to him otp everything just comes back. Plz help meeee!!