Rark

I don't believe there is any trick, and while the obsession may be worsened if you have OCD, there is no real trick for directing your feelings a certain way.

Falling for a friend and trying to remain friends is going to be one of the most difficult and painful things for you to go through I'm afraid, but it doesn't have to be agonizingly impossible.

Like any thought process you want to avoid, don't try and force away the idea because that will make the thoughts that much more stimulating to your mind. Instead, when you get the urge to think of them a certain way, allow yourself to do it. Whenever you get the urge, think of everything you want with them, the great friendship you have, everything you think that makes you a good couple, everything you still think and hope could happen between you. Think of it all, but mourn it all too. Mourn it because it WILL NOT happen. Bawl your eyes out if you have to. If you have someone you can talk to about it, talk their friggin ear off.

It will all gradually get easier. In a way, any large form of heartbreak (especially if this is your first one) will stay with you for the rest of your life, simply because that's the way people work. This isn't immensley horrible or anything. You could and probably will end up with someone who is even better for you and will make you far happier than this person ever could have, but it will just be natural to wonder "What if things didn't turn out this way?", especially if this person stays friends with you. But it will only be a curious pang, that can be brushed away with a deep breath of relief when you think of how happy you are in the present.