i can understand your situation. You have spent a significant time of your life with her and in the process registered a lot of memories. There is no way to delete the memories from one's mind. It's just a matter of time that you will be in sync with the reality. It's in your best interest if you also declare yourself ready to mingle and move on with someone else, that way you will be distracted and the memories might not be surfacing to consciousness to disturb you in present.
Next Steps
Declare yourself open, available and ready to mingle and start afresh with a new relationship.
Health Tips
There is no rule that a person should get engaged or involved in a relationship only once, rather for most of the people it is second, third, fourth or fifth relationship which works. So keep on trying and wait for the person with whom you can stay stuck for life.