I think, it's hard to forget someone who was once a part of our lives. Whatever will we do, it will always make us thought about him/her. But, if he/she chose to leave us, there's nothing we can do. We should not be protracted in the dumps. Maybe this time he/she was happy to laugh at our adversity.
The easiest way to forget someone is to love someone new. However, if we can not open our hearts to love others. We can step into a positive thing. Do the things that once could not do when we was with him/her. Enjoy solitude. Enjoy your time with your family and friends. Try new things and new hobbies.
Hopefully you can pass your day better. Prove to him/her, even though we love him/her, we will not be dropped just because he abandoned.