Oh love.
I was nc with my mum on and off for a few years - she refused outside help (think big 4 bed house, massive garden) and expected me to 'do' for her whilst my own home fell into disrepair. Meanwhile my brother could do no wrong, despite being no help whatsoever.
My relationship with my mum was destroying my relationship with dc and dp. I HAD to take a step back for my own good, and she managed, sort of, martyring herself to cut the grass over 4 days, and roping in several other folk to do what she'd been expecting me to do alone.
I felt the guilt op. You will feel it too. You HAVE to put on your own oxygen mask first. Protect yourself, your relationship with dh and any dc. Then, and only then, do you offer only what you can to your mum. Help is available to her, she can choose to accept it - she is currently choosing NOT to accept it, and she will soon begin to feel the consequences.
It's tough. Be kind to yourself.