here's some more advice even though I agree with what was said above for the most part. There is not much you can do about that girl you have a couple options with her you could confront her which I know would be close to impossible but you could do it.

option 1 - walk up to her tell her to grow up and move past her immature grudge tell her the letter was for her sister not her mom and be sure to use lot's of curse words and scream at her then walk away ignoring any sort of retort she offers.

option 2 - if you havn't done this already stop talking to her and her bi friend thats simple cut off all contact permantently as for the potential girlfriend thing just quick explain to them before the ex interferes the situation and that your ex spreads romors about you due to a grudge.

Additionally since your ex was a true and complete bitch and nothing like a dream girl at all the fact that she can make friends easily with girls you are interested in means most likely the type of girls you are interested in are also bitches who would betray you also. So you should look elsewhere either way there are plenty of awesome girls in unnexpected places.
Good luck man hopefully if everything fails you can transfer colleges where she can never find you oh and delete your facebook or myspace so she can't track you that way either.