Its still a step though and they add up so its important to keep doing them.
I can understand you feel bad about missing out on the parents evening. Think of it like a physical illness though, that would stop you going and you would still feel a bit guilty & annoyed but would very easily accept that its not practical to attend. Mental illness needs to be no different in that respect because its not that you are dealing with a goal here, you are dealing with an unexpected medication issue and think of it just in terms of medication because if you had side effects from any non mental health medication and were throwing up all day...you wouldn't go and that would be just the way it is in anyones eyes.
I don't know how any of that works thesedays but maybe if your child has a form tutor or something, you can chase this up in a bit when this passes?
Don't feel bad about the Diazepam, thats why they give it out to people starting on SSRI/SNRI's anyway. You hang out as long as you can and then you just need some respite. Its impossible to understand how 24 7 constant higher adrenaline feels, its puts anyone on their knees in desperation eventually.
Yeah, I found relaxation impossible back then. I often couldn't understand how I was managing to sleep given how wired I was all day. I found Duloxetine didn't cause me any issues there and in fact, I slept pretty well.
If nothing is working, try distraction techniques, the ones that are more for panic attacks eg concentrate on an objectr very intently for a least a few minutes. Things like that.
Try and follow a guided breathing exercise of some kind. Something which asks you to monitor your chest moving, as it can be distracting due to the focussing.
I really don't know what to suggest other than things like this because I remember nothing worked for me but I was also afraid of trying things for making it worse. If you haven't retreated so far, trying some of these things could help.
Take care.