She sounds like a bully OP and she continues the behaviour because you let her.
If we follow this to its logical conclusion, what's going to happen if you stand up for yourself?
Let's really push it to its extremes here, what's she going to do that's so bad? Is she going to cut you out of her will? Refuse to talk to you again? Shout at you? Ignore you? Write to the paper?
Your feelings matter and she's upsetting you. She either doesn't realise she's upsetting you or, she gets a surge of power when she does. Either or, her motivations aren't important but your feelings are.
Not doing anything is still a choice. By not saying anything to her and allowing her to continue, you are abandoning yourself. Don't you think it's time you spoke up?
You can do this gradually because assertiveness is like a muscle that gets stronger with time.
'Please don't talk to me like that.' Is a perfectly polite way of asking someone not to upset you.
Followed up with a consequence:
'Please don't talk to me like that or I'll leave/put the phone down.'
And if she continues, then follow out on the consequence.
She won't like it OP as she's used to bullying you and upsetting you but her own feelings are hers to deal with, not yours. You will probably feel very guilty and panic and want to sort it all out and then apologise because you've been trapped in this rut for so long. It takes time and that's why you start slowly. You'll actually realise that the sky won't fall down, the world doesn't implode and nothing terrible happens.
If you're worried about ruining the 'good' relationship you have with your mother, I'd ask you if it really is a good relationship if she won't stop making comments that upset you when you've asked her not to.
Give it a go and practice.
'Mum I'm getting those bathroom tiles I told you about.'
'State of you, who do you think you are? Queen of Sheeba?'
'Please don't talk to me like that.'
'Like what? You're so sensitive, you bleedin' snob. Too good for the likes of me now are you?' (Not sure why she's Steptoe)
'I asked you to stop and if you don't I'm going to hang up.'
'Now, don't get ideas above your station me girl...'
You also need to manage the situation so you're not trapped in the lion's den so to speak.