Just been prescribed Mirtazapine, very scared to take it

I've been to see the psychiatrist today and have been prescribed 15mg mirtazapine daily  for a week then I have to increase it to 30mg daily.

​I've been reading some of the discussions on here about the terrible weight gain and withdrawal this medication causes, 

​I'm really scared about taking it now, very unsure what to do

​I have severe depression, anxiety and dissociative disorder and I need something else to help me, I also take 400mg quetiapine every day but nothing helps

​I had high hopes for the mirtazapine but now I'm terrified  

I have been on 30mg mirtazapine for about a year and a half. No weight gain or other negative side effects. As has been pointed out, people react differently to medication. The one thing I can almost guarantee is a good nights sleep.