Letting go of someone

It’s been over TWO years since I’ve seen a friend of mine but still to this day I think about this person all the time. I have no contact with them either. I think about how maybe they might come back into my life. It upsets me a lot and I really don’t know how to let go of them and move on. I’ve gained new friends and people in my life that I love but still feel sad when thinking about this person not being in my life. It seems every time I come close to forgetting this person, they somehow gets brought up and I go down hill again. The biggest reason I can’t move on because I feel like this person is coming to come back. Any advice? I’ve been really been spending time with myself and realized this is something I’m really struggling to let go of.