I agree @UglyNess, it is so, so difficult. The other posters here are right with their wise advice, though.
I only achieved some measure of peace when I accepted that nothing I say or do would make any difference. She cannot accept any view other than her own and that will never change. It still hurts, but that acceptance brought a modicum of calm, which is better than nothing.
Before I realised this I took the route of heavy drinking to dull the pain. That, as you may imagine, did not end well. As part of recovering from that period and the addiction I had some counselling from an NHS support service. A couple of years ago my M revealed she was also having counselling ("because of what you've done to me" - my issues had only come about because of my wicked nature, you see, and wilful disobedience. Not a consideration at all that I might have been 'self-medicating' via the wrong route in order to block pain, flashbacks or torment.). Her counsellor suggested mediation with her, my counsellor and me. My counsellor strongly advised against it on all counts but I agreed. I knew I would get no peace if I didn't. It was helpful, though possibly not in the way intended.
Within minutes the "mediation" descended into a diatribe about my many inadequacies and failings while the look of dawning realisation on M's counsellor's stunned face grew more pronounced (my support worker was unsurprised; he had warned his colleague of what was likely to ensue). The poor bloke kept saying things like "but that's not what your daughter said... you're not listening... that's not helpful...", finally ending with "can't you hear what your daughter is saying to you?!" He had been conditioned to expect in me an unreasonable, abusive and unrepentant sociopath and was utterly blindsided. He did apologise to me privately afterwards. It wasn't his fault.
I'm sorry - this is deeply over-sharing and I apologise for that. My (rambling) point is that here was a setting with responsible, qualified professionals, geared towards finding a place of understanding and progression - and still it ended in M's complete inability to accept any other viewpoint than her own, even in the face of hard evidence that her opinions were based on a skewed interpretation of facts and events, ranting and storming to the point where she had to be firmly told by her own counsellor to stop it. There was - literally - nothing that I or anyone could say or do to change this. That, in a rather roundabout sense, brought its own sense of peace and acceptance.
What I took from this has become something of a mantra for me now - I cannot change how she views me, what she says or what she does. What I CAN change is how I react to it.
So I do that. I work on what I can change, which is how much I allow myself to be affected by it.
I appreciate that this is much, much easier said than done, however, and it's taken many years to get even to this point. It is still deeply hurtful. But I do my best to let it go. If you don't let it get to you (as hard as that is), she's just an angry old lady shouting into the storm with her words being carried away on the wind.
Very sadly, you are right. You can't fix the problem, or even help it. But that doesn't mean that you are forever at its mercy. You can find a new way forward and be strong, despite (not because of) your parenting.
Sending strength and love. x