I do understand Cath. My dad was much the same in his later years (he lived to 88, so your mum is doing well), and I often felt really hurt, and even angry, at his behaviour. At times it made me feel like not bothering with him at all, though thankfully I have a brother who saw him when I just couldn't face things.
Having said that, I wonder, with hindsight into my dad, whether this is a combination of old age and/or the early stages of dementia. Not meaning to scare you, but my dad was diagnosed (eventually) with dementia, which explained a great deal. He was still an awkward, cantankerous old *, but at least we realised there was a reason for that. Actually he always was awkward etc..., but age made it worse.
I have no real advice, other than that. Not sure if it's worth pushing for a diagnosis even, given your mum's age, but it might make it easier for you to cope. Maybe check out the Alzheimer's Society website for advice. I now volunteer with them and can't sing their praises highly enough. Even if your mum doesn't have dementia, it should help you to understand her behaviour, which does sound like the general decline in brain power which happens with very old age, sadly.
Good luck. xx