My Mother Has Been Taking Mirtazapine For A Few Years

My mother has been taking Mirtazapine for a few years and is on the highest dose. She suffered from a nervous breakdown and at first the Mirtazapine started to help for about 2 years but she then started to feel depressed again a year ago and so the doctor gave her Citalopram to take in the morning as well as the Mirtazapine in the night. She also takes diazepam as both drugs are not making her feel better. She is very anxious, lost all interest in day to day life and has had suicidal thoughts. She is not in a good place right now and I am really worried about her. I am concerned that the mix of the 3 drugs is making her worse.