Your mother is lucky indeed to have such a caring son. You are doing your best to be supportive but you may not understand what is really going on for her. It is rare that all doors to an interest like acting get closed. My guess is that the issues around her acting career are a distraction from something else that is harder for her to face. Rather than deal with whatever it is, she places her negativity and anger at the theater.
True to her theatrical nature, your mother is dramatically (and maybe unnecessarily) ending her acting career. The fact is that your state (indeed, your city) has many community theater groups. There are also numerous young people’s theater companies that count on adults like your mom to help kids learn the theater arts. If your mom can’t find a company to her liking, she could start one. She knows how to run a business. She knows how to act. So I have to ask myself why she isn’t doing something to keep her love of theater alive?
I may not understand the total situation, it’s true. I can only suggest that situations are rarely as bleak as you seem to think. For some reason, your creative mother isn’t using her intelligence and creativity to find a way to fit at least a part time acting career into her life. It could be that she got some negative feedback and she is giving up rather than trying harder. It could be that she is making some kind of point with your father or someone else. It could be that she is really tired of it but finds it easier to go down in a blaze of glory than to just let acting fade out of her life. Or it could be something else entirely.
The only way to know would be to ask her and she may not want to burden her teen-aged son with the truth of her discouragement. You could talk with your dad to see if he has an insight that would be helpful to you. If not, all you can do is tell your mom how very sorry you are that she is so discouraged and then get on with your own life. She is the one who has to deal with whatever she is dealing with, not you. All you can do (and it’s a lot) is let her know that you love her and have confidence in her ability to solve her problem.
I wish you well.