Assimilation of Desipramine

I am 52 years old. I began taking 150mg of desipramine when I was 30 years of age. It worked beautifully unitl I tried to go back on it at the age of 40. My body will not tolerate it at any dosage. Since then my body will not tolerate any anti-depressant except for 100 mg of trazadone once a day. I have been told that my chemistry has changed, and that my liver will not break down the medication and it becomes toxic to my body. Can anyone help with an explanation? Is their anything that I can take to break down the medication correctly or is my body missing an enzyme. Please help, I need to be on a decent anti-depressant? Any answers?

Jack