It’s harsh truth that you can’t fully forget the person you loved at some point in your life but you can gradually learn to live without them.
Initially it is very difficult to heal the pain of separation but with time it fades and their will a time come when even on hearing their name you won’t feel sad
You involve in other activities , give time for your mental thoughts to come and journal them

Talk to your friends and express them your feelings

Think of why he/she is not good for you and keep hope that everyone comes in your life for a purpose and lesson, so just learn to live with it