@Sunshinebea I also have health anxiety and panic disorder with agoraphobia thrown in for good measure. I was on 15mg of Mirtazapine for about 1 year. The initial stages were tough I was completely zombiefied for the first week or so but it does get easier. I didn't think I'd even be able to stay awake the first day, I was zonked!
My anxiety also seems to live in my stomach (probably because the brain and the gut are so closely linked in emotional responses.) My stomach is the first thing that feels off when I'm anxious. The butterflies is probably adrenaline rushes because you are in an anxious state.
Sometimes (well most, If I'm honest) I just need to read about a possible side effect for me to feel that side effect or be convinced its gonna happen.
In my experience of mirtazapine the anxiety won't just turn of like a switch, it just slowly fades little by little over a period of time.
The anxiety after the initial period (I'm not too sure how long this took exactly) just seemed to be dialed down once I had been on them a while. The mirtazapine also gave me the ability not to give a shit about the anxiety after a while. I could feel the anxious thoughts but I just had new found ability just to not care about them anymore. I was doing things and going places I never thought I could before the Mirtazapine. I wouldn't have said I was anxiety free or anything at this stage but it was certainly much more manageable. This is probably why I'm sticking with the increase in dosage in the hope I can get back to that level.
I think the citalopram fiasco as you put it is just putting you into an anxiety spiral (totally understandable it must have been hellish) but I think that bad reactions to medications usually happen early on in treatment. Your on week 4 and your still doing okay(ish). All the symptoms you are describing sound like classic anxiety symptoms.
Regards to fitness and lethargy, you can get over it...I actually lost a stone in the summer even on mirtazapine! and exercised 6 days a week. It took a while to build up to that but it can be done. Try to make yourself do some light exercise, usually for me just by starting the exercise was enough to lift the lethargy. The lethargy in general will pass.
Hopefully for you the anxiety will lessen and the calmer feelings will grow day by day.
I must say tho the only (bad) side effect for me outside the initial month or so was that Mirtazapine made me a bit tetchy/grumpy at times. I'm usually quite chilled out and non-confrontational but I could be raging (not in a out of control sort of way) with some people at times if they annoyed me.
Apparently this is a known side effect but on the other hand if you are an grumpy, argumentative sort,they do the opposite and chill you out.