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Past is past. Letting go of it is a skill that can take a lifetime to perfect, but here are some tips. Meet her. Focus on the present. Try and use the present tense exclusively. "Your hair looks nice." "This coffee tastes great." That's because THAT IS ALL THERE IS. The past and the future - they don't exist. They're only in your head. To the extent that you focus on them, you're not present; the alternative is to be present, which means you pressent yourself to the other, and the more you can do this, the more real and authentic you become.
This is hard to do, but a very worthwhile goal. As much as you succeed with it, your world will be transformed. There's another trick to this: abandon all attempts to control others. Exorcise the word "should" from your life. By doing this along with being present, you make space for other people to be themselves. They feel heard/seen, and don't feel attacked; they blossom and grow in your presence.
You may be thinking this is all very abstract, and what does this have to do with your ex-girlfriend? This: just meet her, accept her, be with her. Walk. Go to movies. Make dinner. Shop. Speak of your pain (but in the present tense!) Buy flowers. Enjoy.