You aren’t gonna want to hear this, but the best thing–the nobelest thing—you can do for yourself, for her, and for her fiancee–who loves her every bit as much as you do–is cut her out of your life.
I don’t mean make a big scene, no long conversations, just steadily and consistiently start the process of becoming more and more involved in things that don’t involve her.
You say you love her? If you stay around her, if you remain a major figure in her life and she in yours, you will never, ever, ever get over her, and you will be in horrible, horrible pain.
She will come to symbolize pain to you, and even as you love her you will start to resent her
Because she’s not stupid, she will understand how much pain you are in, and because she’s a nice person, she will feel guilty for causing you pain, even though it’s not her fault. She’ll be in the horrible position of feeling guilty for being happy, and eventually will begin to resent you for putting her in that position.
Until and unless you break this cycle, you will both be trapped in a downward spiral of mutual resentment. You cannot change that unless you get some distance from her. The brain is a funny thing. It can only think about so many things at any one time, and as long as you are seeing her around, your brain will fall into its old habits of thinking of her all the time. You won’t have any brainpower left to think of anything new, to reinvent youself as a person who isn’t defined by this love for her. And you have to go through that process of redefinition. It may take six months, it may take six years, but it won’t start until she quits taking an active role in your life.
After you’ve redefined yourself, after you’ve fallen in and out of love with a couple other people, after you can look back at the person you are now and laugh a bit and cry a bit and shake your head a great deal, then they two of you may well grow to be friends again. After all, she’ll still be a great person. But you can’t do right by her until you’ve got your head on straight, and you can’t do that without some distance.
You need to get very, very busy, very very soon. You will eventually move past this. She will always be an important part of what formed you into the man you become–nothing can change that, because it’s already happened. But there will be others. I promise you—promise you–that in a few years you will be waking up next to somebody very different, your arm snug around them and their hair tickling your face, and for just a minute you will be so grateful that things worked out this way, that you got the chance to experience this wonderful person you never even guessed existed. It’s not an easy road to get to that point, and it takes longer than you think it ought to, but it will happen.