As someone who has just finished a two year relationship, I can only tell you that if you think about how hard she will tak it, it will be very hard for you. Esp about two weeks after you break up, when it settles in, and you start thinking about the other person and how they are, etc. But it sounds like you really are honest with yourself, and are looking for the opportunity to be honest with her, which is a very good thing. The one thing I would suggest to you, though - dont do it in a letter or over the phone. That - to a girl - is just low and impersonal and disrespectful and all these other big words we girls use a lot :) I can only see two things you can do in this situation - either be very strong willed, sit her down (but make sure you havent argued beforehand, or anything like that - quiet, peaceful kinda 'sitting down') and tell her. Listen to her, and tell her again. If she starts crying, you tell her you can talk about it later on, when youre both less emotional, and leave. Because right now, somewhere in her subconsious is the formula 'he wants to leave-i cry-he stays', and thats not good. For some time, that was in my head, and it really is a bad thought to have, and she needs to see that its not true. Which will require you to be strong willed, determined and unwilling to be held back by tears. Hard - but do-able. Sit on your hands for a few days, when you will want to call and see that shes ok,and if you can be physically away (other city, other country), that helps a lot...
The other thing you can do is less abrupt, more tricky and more woman-like in its basis - build up a distance between you two, or do something that will make her leave you. Im gonna leave the figuering out of the details to you, as I dont much like this plan - its less painful, but much more...sly and false and all other things a relationship should not have...
Lots of luck :)