I'm sorry you're suffering so much.
Just for clarification, mirtazapine is not an antihistamine, it's an alpha 2 blocker (sympathetic response) and a serotonin agonist. It is one of the meds currently recommended as first line to try for treatment resistant or treatment refractory depression, along with Effexor and wellbutrin.
What all have you tried? Have you tried the atypical antipsychotics. Even with your heart issues, you'd probably be fine at a low dose as long as you don't have prolonged QT issues. What about lithium or thyroid supplements? I get your choices are fairly limited but there's a lot of meds out there, chances are you will find something that will at least "keep you stable".
Hang in there.