All - it is heartbreaking to hear your experiences and reminds me of times when I would pull the car over , after leaving the Care home car park, to have a good c ry, otherwise I couldn't see to drive!
There is only so long, I think, that you can continue with this type of interaction, before needing a radical rethink, together with all your family and the care home manager. Otherwise you will start to associate these visits with aweful experience and you won't want to go at all. It is a traumatic stress.
With our family, we arranged for other family members to visit for several weeks, so that we could be completely reassured that parent was quite OK.
Another later strategy I found was that, when I was a young 30 something I used to call to see mum and dad but went back to my home for my supper. So I did the same.....I left as the care home were starting to serve a meal. So this behaviour fitted in with what my parent had been quite used to, in the longer past, but also, the arrival of the food service was a nice distraction, with staff busying around, and carers increasing their interaction regarding helping with eating. This worked well. Leaving coat and bag in the managers office also helped .
You could do similar when there are other types of distractions, such as your mum leaving with careers to get dressed, have her hair done, the start of a concert, the start of an organised activity etc etc.
You need to make clear to the care staff that you absolutely need their help in this regard. You can have this added to your mums care plan to formalise thAt they should help with this aspect.
One thing our social worked said to me, and which I found to be true, is that dementia behaviours , those which are tough to deal with, are usually a phase that will pass. Hang on to that, and ensure that you are looking after yourself. You can call the hotline , to talk over your situation, that also helps .