What was your mum's original weight?

It sounds like you think, if there is no physical cause, that her lack of appetite is caused by her mental health condition but that it's not primarily an eating disorder? A number of mental health conditions can exist together.

Do you have any inclination or opportunity to move to your own property in the local area? It seems that her behaviour is very bad for your spirits and potentially for your relationship with your partner who she abuses. I'm not sure how you manage to work successfully from home with all those interruptions but good on you for trying.

Do you feel able to live independently and come into her house a few times a day to help with social and personal care, such as encouraging her to take her medication and eat? What would be your reservations about moving out?

I know that you've tried many things to help with her health and well-being but, unfortunately, there's little you can do if an individual does not want to, or is unable, to take responsibility for it.

Do you feel that seeking counselling for yourself may help you, in terms of being able to off-load the pressure you are under and draw up some strategies to manage the stress and promote your self-care?

I know that there isn't a definitive diagnosis of a personality disorder but go with your hunch. I hope you find the following links helpful which focus on self-care, how to communicate with someone with this problem, how to set boundaries, etc.

https://www.mind.org.uk/information-sup ... ds-family/

https://www.helpguide.org/articles/ment ... sorder.htm

https://www.rethink.org/advice-and-info ... arers-hub/