Wow. What a betrayal. First off, you need to know that you get to have whatever feelings you have about reading that letter. You're in shock, hurt and angry. I would talk to your friends and family and let people close to you support you in your healing from that wound.
As your emotions settle, I think you should also look at her position in this. She has a right to be as shocked, scared and angry about her son's engagement as anyone. How she handled it
is what you have the problem with. If her relationship with you has been good, then I would see this as something that needs to be talked about openly and worked through. All parents react to their children's engagements - some do it well, and others act in ways that they might want to retract with hindsight. She isn't the first mom to have a spaz, but she's your future MIL, and that makes it your problem.
I think it's going to be a pretty rough weekend if you go. Only you can decide if it would be too harsh an action to not go-...how many other people are involved...what level of planning has already taken place....expense involved...things like that. If there are a lot of others around, it's probably safer. If it's just going to be a quiet, intimate time, it may be the perfect time to get it on the table and talk about it.
What my point is, is that you both have strong feelings about an event that may not stay at the hieght they are now, and you're looking at a lifetime of future interactions. Make peace with yourself by knowing who you are and allowing her to be mistaken about you. We all have to let others be wrong sometimes. Where the line gets drawn is what kind of boundary you want to set with how she treats you. It is possible to just tell her that you read the letter and it hurt you greatly, and that you hope that in the future she is able to get over her reaction and to see you as bigger than your disability.
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