Hi @Holly123, and welcome to the community.
The feeling of isolation during withdrawal can be overwhelming, but please know you are not alone. You have found a space full of people who understand exactly what you are going through.
It is completely understandable that you thought stopping at 3.75 mg would be fine. Doctors rarely (dare I say ever?) warn us otherwise! Unfortunately, with these meds, even the smallest doses require a very slow, careful taper to avoid the exact muscle spasms and anxiety you are dealing with right now.
I'm not familiar with Mirtazapine, as I'm tapering off Zoloft. But you will find drug-specific resources on the left sidebar, along with a Mirtazapine specific forum section. One of our members (@Cosmic1973) even wrote a letter to his younger self there about his experiences there:
https://taper.community/thread/a-complete-guide-to-tapering-off-mirtazapine---a-letter-to-my-younger-self-1e4917d8-405e-4d99-894a-98f7fbe4b858
Are you currently holding at a specific dose?
We all want to be done with this yesterday, but it looks like this process just takes time. In the meantime, if we can help, we will gladly try to.