jaded, but eyes wide open
There's a difference between feeling as though you've made a mistake, and needlessly second guessing yourself about the bottom line fact that things feel more bad than good. Mourning the loss of the good aspects of the relationship from your life is unavoidable. After all, if there wasn't anything to miss or be sad about, why would one even be in such a relationship?
Ultimately I think in any relationship you can drive yourself insane viewing patterns of interpersonal interaction as being subject to change, so long as there is love, communication, understanding, patience, devotion. That kind of shit seems really nice on paper and romantic movies but in the real world things just don't work out that way. Sometimes the cold and hard truth is that the effort required from both parties to make things work is just way above and beyond the effort that would be required to derive happiness from a new / different relationship. There are countless people out there in the world, many of them damned beautiful, many of them unique personalities, many of them with good hearts (if I believed in a soul I'd say that too). Usually when we think only our ex-lover possessed these attributes and no one else do things hurt like a bitch.
Unfortunately there isn't a shut off switch for memories, for longing. The core of romantic / passionate love is idealization, so long as you idealize your partner, it'll hurt. It's interesting, a study on university aged males showed that those who scored high on ratings of idealism not only believed more passionately about "love" in life but also had higher incidences and intensities of depression. All you can really do is focus on living your life, redefining it without the other person, trying to fill the roles they singly played through many other people.
The grass can appear green or withered either side, if you so choose to look at it that way. I think that thinking of happiness as existent in only one option and not the other is bullshit. If you can convince or lie to yourself that where you're at is the best place you can be, you'll be happy (whether you're trying to work things out or not). Blowing sunshine up your ass is the cornerstone of optimism, optimism is the cornerstone of having faith in the future, having faith in the future lets you let go of the past.
What to do specifically? Go porn crazy and alternate hands for the sex drive. Fuck someone if you want and don't feel guilty about caring about your own happiness since your ex obviously doesn't. DON'T listen to any shit that meant anything to you guys, just throw it out. Don't get on the self indulgent trip of listening to depressing music, it'll only make you feel worse in the long run. Whatever the hesitancy you may have towards throwing out other reminders of your relationship, get over it and do it anyways even if you regret it initially. Everytime you think of them say "fuck 'em". People can only hurt you if you give them that right. Decide when it's time to just leave shit in the past rather than think about it - what's done is done, that was in the past, you are here now. Get out any leftover anger/frustration through constructive means (physical exercise is great for venting). Meditate to clear your head of the shit you wish you could just bury. Socialize every damned moment you can, and especially when you don't feel like it. Give yourself to others when you don't think you have a damned thing left because it will atleast get your mind off your own feelings. Don't get onto the self pity downward spiral, self pity is the loathsome hallmark of the loser in life. Drugs and booze may just be blinders to your feelings but sometimes distractions are beneficial. Never underestimate the importance of distractions. Work on yourself, make yourself better than the person you were with them so you move onto better things in your life. Tell yourself (lie if necessary) that whatever actions or decisions you did, it seemed the best possible at the time, and that while hindsight is 20/20, all you can do is forgive yourself and take measures to ensure you don't make the same mistakes in the future. Recognize that existence is suffering in some form or another (go team buddha!), and as such all human beings have the right to try and minimize that suffering in whatever ways they can. Let's you go on with your life and not be upset at the fact that your ex is doing the same, with or without you.
It's all about emotional momentum. Breakups can be the trigger point for some of the most dramatic changes in people's lives, whether those changes are for the better or the worse really depends more on how you deal with it than it does what the other person actually meant.