Advice!!! I need advice!!!! Help!!!!

Okay,so ive had a crush on this boy for about 2 1/2 years. He didn’t really notice me until this year, I became confident ..☺️ we started flirting w/eachother during class yk and everyone started to notice and thought we were together I didn’t mind them thinking that 🤷🏽‍♀️ he got offended and said some hurtful things... anyways,he was always on/off with his girlfriend and I guess I was a ‘rebound’ he would talk to me and eventually get back with her.. just so you guys know, I REALLY like this boy 😊 so a couple days ago he told me he was back together with his girlfriend and yk I was sad 😔 but then today I saw him kissing someone I know. (Were not super close friends but we talk) and my heart shattered,I mean he’s not mine,but everyone kinda knows he’s mine? It’s weird,but he’s cheating on his girlfriend 🙄 he always teases me about not having my first kiss,yet he kisses/dates multiple people😒 idk guys,what should I do???