Hi Nailit, just watch what you eat really, read food labels and try to bulk up meals with healthier foods which are lot in fat, sugar and calories so it feels like you are eating more but without stuff which may help weight gain.
I know the feeling, I was put on mirtazapine and it's great, especially for sleep (the main reason my GP changed it). I have a bit of a triple whammy going on with that, pregabalin and steroids on and off which all can cause weight gain - I guess by making you hungrier and craze bad stuff.
I really need to get weight off my joints (got rheumatoid arthritis) rather than put more on so I'm eating lots of salad (nice in wraps with lean ham) and fruit and veg and avoid high fat and sugar foods - it's actually quite scary when you start reading labels! - and make better choices.
I find fruit ice lollies are quite good fillers when you just want something to munch on.
Exercise is good too - am hoping to try and get "swimming" if they can get more movement in my shoulders and hips (more steroid injections in a few weeks) as not being too mobile doesn't help with the weight gain.
If you can though, getting some good healthy exercise may help both that and even boost your mood more.