So me and my ex have been on and off for almost 4 years ( We got together at 16 we are both 20 now )

He was my first boyfriend, first kiss ect. We have a bunch of history together anyways we haven’t dated for a good while but we still have conversations here and there and 2 weeks ago he messaged me wanting to meet him so we could talk and then we had sex we’ve barley talk since then an 2 days later he was with another girl any time I try to talk to him he opens it and leaves me on read what should I do I love the boy and I don’t wanna give up on me and him but out of no where tonight he blocked me on Instagram and now he blocked my number to but im still on his Snapchat I don’t wanna loose him he’s my everything how can I get him back?