I am so sorry that you received such a troubling letter from this unknown person. I'm sure it is very upsetting to you, and even more so that your father would disclose your address to someone unknown to you. Please remember that this man is behind bars and cannot hurt you in anyway right now, and no matter what your father may or may not have told this person, is not a reflection of who you are right here, right now.
Take some deep breaths and put the letter up (or throw it away) so it is not a constant reminder to you. I know this is extremely difficult, but try to focus on your surroundings right now, and get back to what it is that you were enjoying so much about today. Focus on the good here and now, and let go of the negative feelings regarding that letter. No one should ever have the power to turn your day into something bad, especially someone you've not met.