Getting off of Amitriptyline to start antidepressants?

I've been on Amitriptyline for 6 years, and now take 50 mg daily. It doesn't help as much anymore but it's better than nothing. I was recently prescribed Fluoxetine for depression. The doctor who prescribed it said that I would have to lower my Amitriptyline to 25 mg to take both medicines together, but the pharmacist said that I shouldn't take both medicines together at all. Has anyone had to stop taking amitriptyline in order to take antidepressants? If so, what was your experience? It's hard choosing between my declining mental health and having urinary frequency to the point of going 15 times a day.