How I let go is to plan each day, each hour, each minute with something else
and "reprogram" my free time with things I like, are good for me, are absorbing.
Everytime I catch myself thinking of the person, I make a conscious choice to refocus on
what I'm doing, or something happy from the past not related to the person.
Once a day, or a week, or month, I go ahead an fully feel the pain and grief and
cry or yell or whatever for a short time, and then I stop, wash my face, and return to my pre-scheduled day.
You will find the time you are thinking about him getting less and less as you do this.
When you are ready, during one of your grief sessions, make yourself an affirmation
which is about closure and forgiveness. Start saying it each time.
Reduce the grief sessions as you get stronger, and believe it or not, eventually you will
find peace and closure--not face to face like you wanted, but peace and the readiness to move forward.