L,
I have been in this exact position. I feel like I could have written this myself five and a half years ago. I was in my first serious relationship for 3 years until we broke up in April 2009. I was heart broken for the longest time, we too had to cut all communication because he was quickly seeing some one else..and just as you said, some one I had my suspicions about while we were together and he denied that it was anything but mere coincidence. About a year down the road, they were broken up (she wasn’t such a nice person, it turns out.) A year and a half down the road, I began a wonderful relationship with the man I am now happily married to. It is a much much much healthier relationship and everything I’ve ever dreamed of. Yes the first few months are especially hard, and especially after being with some one for years the pain doesn’t suddenly go away in the blink of an eye. You will probably find out, now or down the road, that he’s not as happy as he is letting on to be with this other girl. My ex eventually admitted after they broke up that he always felt horrible about what he did to me and how I was the nicest and most supportive person he ever dated. Point is, by the time he realized that…I found out that I could be treated much better…and to this day I am in the most amazing relationship I’ve ever witnessed. Although right now it seems like almost nothing will make that hole in your heart go away, and picturing him with her makes you shudder (it made me shudder, too) there will come a time..maybe even not so long from now, that you will find out it really did happen for a reason..and your real prince charming was just waiting for you to be ready to be rescued.

I go through this too.
Sometimes more often, some times this happens once a week.
It feels like I’m attacking myself but I have no defense.
I also battle with extreme anxiety, panic attacks, ect and they seem to tie in together.
I don’t have a lot of advice for you since I’m going through some thing similar but just know you are not alone.
Know you are worthy of love from yourself and from others
<3

Wow, you are not ready to be married.
I can see that she, your fiance, hurt you very bad, I am sorry for that…but if you are still hurt to the point of writing a letter like that to her you are not ready at all for such a commitment.
It sounds like something you’d write to someone upon breaking up, or to your worst enemy..not to your fiance before your wedding day.
The problem here is, these are your true feelings about this woman.
I agree with Josie that it sounds like you want “the love of your life” to feel pain.
Not only would I not give her this letter, I would not marry her..not with feelings like that hidden inside.