Year-long crush makes my life miserable

I've had a crush for a year now on someone who is in a relationship... He has been with his gf for almost a year and a half. I didn't know he had a girlfriend when I started catching feelings for him but they kept getting stronger even though I found out. He has definitely flirted with me on numerous occasions (we see each other a lot due to common friend groups), he locks eyes with me, touches me "accidentally", compliments my appearance, we can talk for hours, etc. We went out one night with other friends (without his gf) and danced together while he looked at me in the eyes. When he's with his gf however it's as if I don't exist, he won't talk to me, or if he does he won't look at me. He also constantly kisses/hugs her in front of me - especially when I am talking to her (he interrupts our conversation to hug/kiss her)... I don't know why I am so into him, I feel like the chemistry between us pulls us together, but I am also mad at him for sending me mixed messages. I have tried to stop because I feel stupid but I just can't. Every time I go on a date with other single guys (really nice guys) I just end up missing him and wanting to leave them and go to him. I have recently been very careful not to do anything to encourage him, but I still want his attention so much. We also run into each other all the time which is annoying. I sometimes think about letting him know about my feelings although I am 99% certain I'll get rejected. I don't get why he'd flirt with me since he seems so happy with his gf (since he constantly hugs/kisses her in public)