I can only guess why you feel this way, but one reason may be that down inside you still loved her, in spite of your difficulties. After all, at one time you did love her; otherwise you wouldn't have married her. Perhaps the embers of love still glowed deep in your heart, even if you weren't aware of it.
But I suspect you may be experiencing two other emotions that cause you to miss her. One is regret -- regret that your marriage wasn't better, and regret that you didn't try harder to bridge the gap between you and rekindle the fire of love you once had. Perhaps your letter will cause someone else who's going through a hard time in their marriage to stop and think what they might do to reverse their downward spiral.
The other emotion you may be feeling is loneliness. Even if your relationship was rocky at times, you still had each other. One of the reasons God gave marriage to us was because He knew we needed someone with whom we could share our lives. At the beginning of the human race, God declared, "It is not good for the man to be alone. I will make a helper suitable for him" (Genesis 2:18).
My prayer is that you will turn to Christ during these days, and discover the joy of His peace and presence. You can't change the past -- but God loves you, and Christ can give you hope.