How do I get past someone I love leaving?

Especially someone I love who I am not supposed to love, or at least I can’t tell him that I do.

The details are complex but the situation is he is involved with someone else and I am too, but I cannot get him out of my mind and I feel so guilty because who I am with is so wonderful and brilliant and everything I could ever want, but then he comes along and my whole world has been turned upside down and I don’t know what to feel anymore.

At first he was just someone I flirted with a lot, someone who was just easy to be with without any challenges or problems, gave each other an ego boost, but it grew and we got closer, a bit too close but never over the line.
I guess my calling it love is too far, but it is more than infatuation, less than love, so I prefer to call it love.

At the end of the day he is just someone I at the very least always want around in my life, regardless of whether or not we are together. But that can’t happen and he is moving due to his job halfway across the country and I may never see him again. But I can’t stop him leaving, I am just another friend, nothing more.
I always hoped that maybe he thought of me how I thought of him but it was too risky and stupid to find out.

All I want to know is, how can I get past him. How can I spend my time after he has gone NOT wondering what he would have said to me today, how many times he would have smiled at me, how he would have made me laugh, what cologne he would have worn, where he is right then, what he is doing, and if he is thinking about me or if he is happy or sad.

How can I get on with my life? He is gone in about a month and then that is it and I am milking every second I get with him. But our relationship is nothing so much it would stop him leaving and if it were the consequences would be chaotic.
I cannot imagine life without seeing him.
Would I bother even dressing up/putting on make up/looking nice, being in a good mood, or anything if he wasn’t around to see and appreciate it?

That scares me because this is my life we are talking about.
Please dont tell me to talk to him about it because I have decided to let him go and move on, I just need to know how.
I have never EVER lost anyone in anyway before, all my friends and family have always been around, this is the first time anyone I care about has left me. I guess I am blessed and cursed in that way.

Please tell me your own experiences and how you did it, if you ever managed it. Did you just cut off your feelings, or did you enjoy the time you spent together in your head, addicted to nostalgia, appreciating the fact you were fortunate enough to have ever spent that time with him/her?