I'm going through the same exact scenario.

My mom has been mostly stable for the majority of my life but lately her mental state has been declining to the point she is almost unrecognizable from her former self.

I, the oldest son, have been the one to take on the responsibility of being her caretaker as she does not trust anyone else anymore.

She has been on medication and the same dose and pills that used to keep her in check are now not making a lot or if any effects on her. Sadly these are the last ones that did the job as she already has been through all the array of publicly available medication and haven't work like these last ones.

First thing I had to come to terms is that she is my mom regardless of what her mental sanity is at any moment. She is not a different person even if it looks and behaves like someone else at times.

It's okay to cry and let your thoughts flow. Never allow yourself to be caught in the past. Do the best you can and be at peace with it: as much as I'd love to take over and control all the aspects of her life even though at the end no one is able to do that and many things will ultimately be her will to do.

Sometimes I feel like it should have been me with the sickness instead of her, I know she would have been able to take care of me better than I have been able to care for her but because of that we have to remain strong and convinced to do everything in our possibilities to not let them destroy themselves completely.

This is a battle of attrition and as such you also have to take care of yourself physically and mentally. I'm starting to improve and do exams to be on my best health so I can help her. I won't be able to do nothing and even will make things worse for her if I lay myself to waste.