Howd you get rid of someone in your mind after being with that person for four years but still have fealings4?

ive seen this person and you can have these great feelings about them but its like they r scared to break open you see that person laugh and have a good time and then he/she just shut you out its like whats up with that now ive finnally realized that i have to move on but that person is still in my head how do i forget or just get him out of my head...?