@LA in Boston,
LA,
Many of these respondents are correct about the big things, in my opinion, but I hear your strong desire not to abandon your parents. There are correct and proven ways to deal with your issue, and these people are advising you to follow those ways.
It is to your credit that you want to help your parents, and that you're willing to put up with some abuse in order to help them. You must also realize that living under constant stress and abuse will change who you are over time in ways you may easily realize - and in some ways that you don't notice, but others people will. The worst thing is that when you enter a relationship and become a parent, you may pass this toxicity along to your partner and children. So, the people trying to get you out of the house know your risk even if you don't.
If you want an alternative attempt to leaving the house now, the first thing I'd do is speak to all your siblings about the issue. See if you can get support from them in helping your parents. Start with moral support or being on the same page: Do they agree your folks need therapy? Would they be a part of a thoughtful intervention to get them in therapy? What is the extent of their willingness to help? Inform your siblings that you are on the verge of moving out, and their help now in intervention may help them avoid financial help later if you're forced to move out.
I hear from your narrative that you're the only sibling trying to do the right thing, but before you go unilateral on mom's ass, make a REAL effort to get your sibs on board - at least in a supportive role - is job #1. One is better than none. You shouldn't bear this burden alone.
While you work on this with your sibs, think about documenting or recording your mom's abuse. Get a notebook or an app on your phone to record. She needs to hear herself. Plenty of people aren't aware exactly how abusive and cruel they are until they hear themselves. Get as much of this as you can to prove your point to her. How and why coming up later.*
The only additional thing I can think of to make a change while you try to get the intervention together is how you respond in the moment of abuse. She should notice your mantra and your altered behavior, I think.
Have a patterned response to her abuse, depending on how bad the abuse is.
Are you up to totally turning the tables and being assertive? Is this someone you can be?
1. Mom makes mild sarcastic remark that insults me:
Stop what you're doing, face her with your entire body, mindfully stand tall, look at her until she looks at you. (<---I think this is important.)
Say something to reject the criticism:
" I don't deserve the sarcasm (or the insult.) I know you're in pain, but attacking me is not going to be how we deal with it any more. You're going to have to communicate to me without the sarcasm (insult / yelling). (Pause) (Try to help train her by then lifting the tone of your voice and saying) -
"Did you want to ask me / tell me something?"
Tone matters. I hate to say this, but training people is a lot like training dogs. The tone you use to respond to her verbal abuse should be standing straight, coming from a place of firmness, self-respect, and personal strength.
Then, you offer her a way out (a way to save face) with the last sentence. If she's enraged at her own pain, she'll likely respond to this change in you (a challenge) with increased insult. So you lighten your tone and maybe show some acceptance raise your eyebrows or something and ask, "Did you want to ask / tell me something?" You're remembering she's being controlled by pain.
The rancor and challenge are no longer hanging in the air between you - a kinder offer to listen to her is. This method also negates whatever she said to you. You didn't hear it, and require it to be said again - hopefully in a respectful way. At any rate, she is less likely to repeat the sarcastic or abusive statement twice - after you've made a point of rejecting it.
It may seem like potty training a child or mat training a dog - very repetitive and seemingly without success for a long time, but you are training yourself too - not to accept bullshit and be poisoned.
*2. If the abuse is particularly bad - if she says something that hurts you to the soul:
Stop what you're doing. Go get your phone or notebook designated for this purpose. Return to her, assume your standing tall stance. Don't say anything, just begin writing down everything she said and date it. If you have problems remembering her quote (it should be ringing in your ears), ask her.
"If you weren't so lazy, you wouldn't be so fat." Is that what you said, Mom? "Let me make sure I don't misquote you, Mom. You said: "How will you ever find a man if you spend all your time watching TV." Let her hear her words and know that you're now recording them.
Every time it gets this bad, after you finish recording what she said, I think I'd say something positive - like "Mom, I'm here to help you and Dad, and attacks like this are poisoning me. I can't stay if you don't stop."
After a while, you'll have a pretty tidy catalog of mom's crap. This is useful for an intervention if any of your sibs are wiling to help. Mom needs to hear what you've had to listen to - and dad does too.
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Another ploy: Earplugs. Keep them in your pocket.
Mom starts bitching, hold up a finger "Just one minute!" Pop the plugs in. "I'll listen to you when you speak to me in a way I deserve to be spoken to." Hope they're good plugs. Get your dad a pair.
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Another idea. Go to her when nothing is wrong, and ask her if she hurts. Offer a rub. Hands on your body can alleviate some forms of pain sometimes. Maybe even research hands on therapy, share the info with your mom and your dad - and touch her and massage her. Sometimes dogs can alleviate pain and lift spirits. Is there a traveling dog therapy thingie in your area? A lap dog you can borrow from a sib or neighbor for her to spend time with? Get your sibs and dad reading up on holistic therapies and let mom know you are all doing it. It may really hearten her.
Good luck.