Sorry to be late replying. Aphasia can be so frustrating for the person affected and the caregiver trying their best to understand. I’ve included some links that might possibly be useful.

How to communicate with a person with dementia | Alzheimer's Society

Communication challenges and helpful strategies | Alzheimer Society of Canada

Having said that, you could and indeed might already follow the communication advice perfectly and still get nowhere. I hadn’t heard of speech and language pathologists before (over here we have speech and language therapists not sure if the same) but I’m curious about that, might be worth investigating?

Lastly if it’s appropriate shared humour can go a long way. I normally laugh at myself when I can, it certainly calms things down.

Don’t be hard on yourself this stuff isn’t easy.