Hi OP, I just have a diagnosis of anxiety rather than GAD so I'm not sure of the differences but I hope some of this might help.
I also tried citalopram and mirtazipine, I had awful experiences on both of them, so you're not alone in your experiences of them. Citalopram gave me awful nightmares and upset stomachs, I put on 3 stone with mirtazipine and it definitely made my anxiety much worse, even though my consultant kept on saying "it must all be in your head, mirtazipine is used to treat anxiety" which was helpful. I found 5htp made me worse too, quite jittery.
In my anxiety tool kit is meditation (I know you've tried it but try to stick with it if you can), emotional freedom technique, hypnotherapy, yoga/stretching, yoga nidra (which is really just a deep relaxation technique) and other deep relaxation techniques like progressive muscular relaxation twice a day, walking 3 times a day after each meal (helps get adrenaline out of my system).
I work but part time hours, and to be honest I feel like I don't have a lot of life outside that trying to manage my symptoms sometimes and I do a lot of avoidance of things I know will make me worse (the news, certain people, certain situations) which I know is counterproductive in the long term.
I've recently got part way through Feel the Fear and Do it Anyway, which I think has helped a bit. I also read the first Claire Weeks book a while back, a bit old fashioned but helped at the time.
I hope that you can find something that works for you, whether it's medicine or non-medicine based, also hat off to you for working full time with GAD.