Please help me to be kinder…

How is her eyesight? If it's good enough to read a bit, texting or emails might be helpful for conveying important information, subsequent phone calls may go better if she has an idea what you are likely to be talking about. Or something like FaceTime might be better than a phone call, people do naturally lip read to some extent if they can see the other persons face and that might help her understand?

Your Mum won't change

You can choose your reactions to her and to what she says

That's as good as it's going to get

Agree. My dh has hearing loss and won't wear aids because it just amplifies all sounds. He's not an idiot - would wear them if it helped.
I totally understand your frustration though OP!

I had a difficult relationship with my mum. Her own childhood meant she became a difficult woman. The last 6 months of her life after having had a catastrophic stroke were appalling for her. I pulled out all the stops and am so glad I did. For my own peace of mind as well as the ways in which it helped her.
I would urge you to think about that - your own future peace of mind at the very least. Guilt would be a horrible feeling to deal with.
Frankly, having to shout on the phone a bit because of a stubborn refusal to wear hearing aids is small fry compare with what many people have to do. Old age can be brutal.
Obviously have a go at getting her to wear the hearing aids. Try telling her that dementia is a big risk from deafness. If she's pin sharp that might sink in. Flatter her - say 'mum, you're so sharp and so many people your age have problems. I'd love you to wear your hearing aids to help prevent you losing that'.

Are the aids properly maintained? New batteries/recharged every week? Serviced regularly, moulds fitting correctly? They can be very uncomfortable and depending on the cause of hearing loss, not always that effective.

I'm not sure that people realise how isolating deafness can be, couple that with cognitive changes as people age and it is a difficult combination to cope with.

There also comes a point when the relationship changes and roles reverse. Your parent can no longer be to you what they have always been. It sounds like you may have reached that point. It is frustrating and upsetting, but it is not their fault. They are probably also feeling upset and frightened at the change and their decline. Try to bear that in mind and compassion will follow. It's not easy, but comes with practice.

To be fair to your mum - hearing aids can be a bit useless IME. My dad has hearing aids and he does wear them but it's still really difficult to have a sensible phone conversation with him.

What sort of hearing loss does she have?

I'm mid-40s and have genetic mid-range hearing loss. So I can hear high and low pitched noises very clearly, but mid-range sounds (human voices, annoyingly) are very muffled. I have hearing aids. I hardly ever wear them. I was so excited to get them but they don't help very much as they amplify all sounds. So when I wear them I can hear all the high and low pitched noises REALLY LOUDLY and still struggle to hear what people are saying (unless I am in a quiet room with no background noise). Hearing aids aren't a magic solution for everyone. They can also be fiddly to put in and uncomfortable to wear after a while when you are first getting used to them.

I didn't realise any of this when my Dad started to wear his (for the same issue), and remember being annoyed that he didn't use them more!

Aunt is exactly the same conversations over the phone are really difficult. There are phones that are for the hard of hearing.

Ive had a difficult history with my mum and I find it hard to be kind and gentle. I feel guilty and angry at the same time. Awful

Dont have any advice OP but I understand

I’ve found that I have to restrict what I say to my DM. Certain things we talk about will make a response that I do t like, or don’t agree with, so I just don’t go there any more. I try and think of easy things to chat about while I’m there,

You need to figure out why she won’t wear the hearing aids. They might not be working properly. All the kindness in the world won’t enable a proper phone conversation if she can’t hear you.
Been there with both mother and MIL. You need to be like a broken record asking her if she has got them in. If not call her again in 20 minutes when she has.

My mum will wear 1 hearing aid, but not 2. She says she hates the feeling if being 'shut in' by them.

Nothing helpful to add, sorry.

Why won’t she wear her hearing aids? How long has she had them and has she ever worn them?

Presumably she’s had a hearing test at some point and been given aids due to a hearing loss. So she has some awareness that she has a hearing loss. Do you remind her of this?

i guess at this stage you either put some time and effort into getting her to another hearing test, or to start wearing the aids she already has or you accept that this is likely how communication is going to be from now on, and you adjust your expectations accordingly.

she is now an old lady who needs kindness no matter the history.
Really? If you - for whatever reason- have a poor relationship with her, it’s going to be quite stressful to pretend otherwise just because she is now old. It’s not clear in your post what your expectations of your mother (and your phone calls with her) are. Is she someone you want to talk things through with? Are you looking for compassion and support - which she can’t give when she doesn’t know what you’re talking about? Or are they ‘duty’ calls - which you could make shorter, less frequent and avoid discussing anything sensitive?