Well I'm dealing with a lot of obsession over an ex myself right now, and I won't say I am over it. But I will say that for me at least, giving myself some kind of ultimatum to stop thinking about her doesn't seem to work. The best I am aiming for right now is to not neglect other activities and values in my life that have nothing to do with her. And reaching out to other people seems to be key. And not neccessarily reaching out in ways that have anything to do with her either, just having some human contact so you don't see your ex as the only person you can relate to in the world. Because for me anyway, I can get really isolated and into tunnel vision where I feel like no one in the world can really get me or relate to me like ___. And it's true no one can replace anyone else, but it doesn't mean I only need one person ever.

Also when I can just feel love for her and want her to be happy, though it sounds all "selfless" it's actually something that makes *me* feel better, more than anything. Those moments when I can really feel she'll be happier not with me than she could have been with me and that that is ok, are the best moments for me. I just feel like we are both freer and it takes me out of my own pain about things to just wish her well.
And just because it couldn't last doesn't mean the love wasn't real, just there's more to being with people than love.