I feel your pain. I had a spotless record for over 20 years, my PM didn't fire me but she sure tried for about 4 years and yes it can get old quick and the abuse can get unbearable. I will tell you what I did.
The first mistake I made was sucking it up for much to long, didn't complain or tell anyone outside the local PO, if I had it to do over I would have started filing EEO's much sooner. It is amazing how things start to happen when it starts to cost the Postal Service money.
Document Document Document, call the 1-800 harassment hotline (every District has one), during the National Convention the PMG told us to report abuse to her office, I would write a letter and Document the abuse some more. Call the EAP and make an appointment for Stress this cost the Postal Service money. File a EEO. File a Grievance and cite MOU 14. And if your office doesn't have a local Union steward, well you already have a target on your back get yourself elected.
The PM in my case had lied to the POOM so much about me they thought I was the devil, I had the title of trouble maker and was on my way out the door, I did go out the door but it was to hold it open for the worthless piece of crap PM that totally upended my life for 4 years.....PM is gone now. Please keep yourself calm. There is life after a abusive PM.