Mom is 85. She is constantly using the restroom and by mid day is completely exhausted from all the walking back and forth. I feel it's partly psychological but could there be a physical problem? The doctor says she isn't constipated. X-ray and CT scans are good.
So ask your Mom if she is going because she feels constipated, or is she have some urine frequency. You can order plastic "hats" that you can put in the commode to check how much urine is being eliminated with these frequent trips. There are some medications that can lesson the urge to go if this is necessary. So I would continue of the medical workup. Get that U/A done. And if there's no problem there, discuss with Mom why so frequent on the trips, measure output and times for 24 hours, and then off to a Urologist.
I often say that if it wasn't for the need to use the toilet, elders lives would be SO MUCH easier in general, right? Both my parents were fixated with the toilet, and my mother would wind up falling off of it frequently as her dementia advanced, to the point where the caregivers would find her on the floor of the bathroom *in her Memory Care ALF* at least once a week. Thankfully she was checked on frequently, so she never laid on the floor for hours on end. Sigh. She too felt the need to go to the bathroom all the time, but she was wheelchair bound AND in Depends 24/7. Dad hated to actually urinate IN the Depends, so he was constantly wheeling himself back & forth to the toilet too.
I see you said mom isn't 'constipated', so is it BMs she is constantly feeling the need to have? You say she has dementia & mental health issues also, so that could be what's causing her this problem: psychological matters. If her medical scans are good, all you can do is give her more fiber to eat, but then she can wind up with diarrhea which is no good either. My 'uncle' George who's nearly 102 is on hospice and going thru this exact thing right now; but he hasn't had any tests to determine if he has a blockage. He's in the bathroom for hours on end 'trying' to have a BM but nothing happens. He eats very little, also. So there's no answer for him, either, sadly.
I hope this matter resolves itself b/c it's very frustrating for all concerned. There's nothing 'good' about dementia, let's face it, but there are situations that make you want to rip your hair out, I know. This sounds like one of them.