I switched from 150mg Sertraline to Mitrazapine at the end of July. Initially 15mg, now up to 30 with Dr recommending we wait another couple of weeks to up the dose again.
Like you, I felt horrific in the first few weeks. I could not function at all, was like a zombie.
I'm now in week 4 and they side effects have greatly reduced. Persevere if you feel you can but if it's too much go back to Dr ASAP and ask to try something else.