Yeah, it sounds like you really need to let go of "trying to change my mother." If she chooses to be miserable, it's not your responsibility. I'm sure you know this, but you really have to FEEL it. Hopefully therapy will help you get there. Perhaps you can also practice certain "automatic" responses that you can pull out in a pinch. For example:
Mother: I can't believe you're going to the wedding.
You: Thanks for your opinion, mom.
Mother: Well, so-and-so is the maid of honour and she's living in sin.
You: Thanks for your opinion, mom. I'm going to the wedding.
Mother: Do you really want to support this type of ceremony?
You: I have no problem with it, but I thank you for your opinion.
I'm so sorry you have to deal with that along with the problems in your marriage.
I cant really give you mother advice, since I cut mine out of my life 30 years ago. (dont regret it one bit)
...I think you should practice the exercise that FLM suggested over and over...and, I think you should go back through this thread and rewrite (on paper....really, trust me...) each thought about your Mother in which you've given her power..Instead of "she it manipulative and..."...you can write..."I allow her to manipulate me"...instead of "she makes me so made"...you can write "I allow her to get into my head AND heart...she belongs in my heart as she is a grown woman who is not asking for my help, she's asking for to control my life...only I can give her that control"...if I were you, I would not allow her into my life...let her skim the top, the surface...kids, recipes, what's for dinner, how's dad (but, don't engage her in criticism about your dad...or anyone else or you've opened the invitation up again to allow all of the manipulation and control back in)....you are in control here...I don't know how old you are, but it's time that you accept that she isn't going to change...and, by the way...you're acting a great deal like her (in an internal way)..she's probably thinking "if my family members would just change their ways a little bit..."...so, accept who she is, draw boundaries (FLM's exercise) and then take care of yourself...
I would never advise someone to cut their mother out of their life.