How do I stop missing him

It has been a month since we broke up. I went on vacations and enjoyed myself, but then I start missing him. Every little thing he said comes to my mind, even though it wasn't my fault. I didn't want the relationship to end. I sleep thinking about him, and I wake up thinking about him. No matter where I go, I can't get him out of my mind. There are so many memories between us; my heart doesn't want to delete them. I can't believe that he has really left. Now I have to go to university where we shared so many memories. I am thinking about how I will manage.