Switching from sertraline to mirtazapine

So 2-3 years ago I came into a dark patch, anger issues and depression used to smash things and all sorts. Anyways after starting 25ml of sertraline I felt no real effects I went up to 50 and then to 100ml where I stayed on for 2 years and last year I became severely ill with stomach flu which I think caused me to get severe health anxiety Which also turned it into generalised anxiety year passed on of therapy and my anxiety would come and go until about 1 month ago it got worse my doctor prescribed me a extra 50ml of sertraline so 100 in the morning and 50 at night but I started to notice that I would get more anxious 20 mins after the pill which causes nauseous feelings, head aches on and off, heart palpitations shortness of breath ect, over a year and half ago I was having the same symptoms but wasn’t anxious,I had many ecgs and a lung function test and all normal bloods normal ect but after this extra dose its opened my eyes that maybe alot of my symptoms are intact the drug sideaffects so I mentioned to my doctor and he wants to stop taking it and start taking mirtazapine 15ml in 1 weeks time. I’m really afraid of the withdrawals as I have a phobia of vomiting. Anyone actually came off sertraline onto mirtazapine. Was it better? Did it not work ect? Share me your experiences please