Most Helpful Girl
Depends on what she did for you two to break up.
If she hurt you to a point where you cannot physically handle to be in contact with her AT ALL, then don't.
But I you think you can manage to read the letter without over reacting or getting too upset, then go ahead; everyone deserves a second chance.
You don't have to date her again, but it's always nice to not have a grudge against anyone. Maybe you two could become friends to some extent.
Most Helpful Guy
it depends on a lot of reasons, but the result is same and the reasons are multiple.
tell her that you forgive her before she send that letter+thanks+good luck.
that's will make her feel guilty for what she did and will show her that you're the bigger one with a lot of self-esteem. (strong personality)