Hi and welcome to the forum.
I'm really sorry you are experiencing this, it sounds horrendous and must be really frustrating for you.
Here is a link you might find useful from Mind, about how to tell the difference between withdrawal and becoming unwell
Mind - Withdrawal symptoms
I can relate to what you say about feeling your brain has turned to mush. It's difficult to know what the cause is. I found antipsychotics reduced my mental acuity but then I think my depression does this also and I'm never sure which is chicken and egg.
Can you get a second opinion off another doctor?
I think there is a section on this forum about help with coming off medication, there may be posts there that can help.