So my ex has a girlfriend but I still want him

My ex has a new girlfriend and we broke up just after Christmas and I can’t stop thinking about him and I see him all the time and we have a laugh and everything together and he’s always looking at me but I can’t just tell him how I feel cos he has a girlfriend and that wouldn’t be fair on her. Like before they got together he would touch my hand when watching stuff and make me sit near him then we didn’t see each other for 6 weeks or talk and then I find out he has a girlfriend. I wish I told him how I felt and I kick myself cos I shouldn’t feel this way about him because it’s been so long we haven’t been together. Plz help what should i do. I can’t think of me with anyone but him he is just my best friend I would go to him to make me feel better and I still would