Keep busy. If your mind is occupied then you don’t have as much free time to think about him. Surround yourself with people that make you feel good. Do things you enjoy. Plan something to look forward to. Delete him from all your socials. You do not need to know what he’s doing nor be tempted to message or call him.
But most importantly, give yourself time. Because above all time is what it will take. You’re grieving and it’s ok to feel sad about the end of your relationship.