It is difficult yes. And it is easier said than done, but I consider myself rather strong mentally in this aspect, even though I dont know why he is still on the mind (even if so negatively).
Why dont you please think like this? He cares about someone else, he chose someone else, he wants her not me, he cares how she feels not how I feel!...
If he places importance on another woman over me, that is when the doors are permanently closed off from your end too.
I dont like alphafemale in general coz her prediction on a simple immediate career Q didnt manifest, but she had once told me that I wont let him behave the way he does, and will not tolerate the nonsense- she was so right on that one.
When he was willing to demote me down to only "friends", and became defensive on being just that, I severed all forms of contact with him. I put my foot down, and did not allow the nonsensical behaviour from him. By friends, he could have had the option of having many women present in his life as "friends" without having to commit. Why would I have allowed that?
That said, is he not on my mind? He is, sadly, but I do not allow positive thoughts i.e. all I think about is he cares about someone else (which I suspect he does), he does not think highly enough of me to pursue me (and thats his choice).
If he has to be in my mind without my control for the type of connection it is, I will ensure I try best to drive him away, by thinking negative. Why allow myself to be tortured by this feeling of him pursuing someone else?..