There really is nothing you can do other than "accept" the relationship is over.
Why would anyone want to be with someone who does not want to be with them???
Having said that do not be surprised if she contacts you (after not hearing from you for a month or two). She'll start to think maybe it was YOU that broke up with her or at the very least you were just as ready to end the relationship as she was.
Getting dumped can be an ego crusher and for others dumping someone (who seems not to care about being dumped) is also an ego crusher.
I recently wrote a hub about getting over breakups.
The bottom line is people break up with mates according to (their comfort level). So don't get caught up with the method that was used (in your case a letter).
Keep in mind whatever her reason for letting you go could be the same reason why the next woman will fall madly in love with you! Making changes to fit into your ex's idea of what you should have been is a waste of time and energy.
Accept the relationship is over.
Spend time with your family and friends
Get back to doing things/hobbies or activities you neglected while dating.
Rebuild your self-esteem (self help books,workshops...etc)
And when you ready get back out there and start having some fun casually dating. (Don't put any pressure on yourself to find "the one"). Let life unfold naturally.
Best of luck!