I like the letter idea. I was going to mention but someone got here first. So, I will just say that writing some letters to my dead great aunt helped me to feel less guilty about my failure to care for her in the way that I should have when I lived with her. This letter idea is a way to symbolically express your feelings to this person. Keep in mind that initially (for me anyways), the guilt gets stronger because you are expressing it. So, you might want to gradually work your way towards expressing it.
Do you have a friend, pastor, counselor or someone that you can talk to about this? Anyone who is a good listener can be of some help. You might consider talking to a counselor because Ts make great listener and can sometimes give you good advice.