"Mom, have a good time at the convention. I know you well enough to know how much thought and care it took you to write that text. I also know enough to know that you are feeling guilty or responsible about my not being at the meetings. You are a great parent, and should not take my not coming to the meetings as a reflection of you as a mother, servent of Jehovah, or otherwise. I made my own decision to not attend, based on my own hours and months of personal study. I love you too."
You have to define who you are within this relationship with your mother. If you don't nip it (her guilt trips) in the bud, she will continue on & on & on & on & on.