Thanks a lot for the replies, guys. I find it really helpful to get a different perspective on things when my head goes funny.

SoupDragon, no, I currently haven't got a therapist, as the previous counselling I had was very time-limited so that finished quite a few weeks ago and I'm waiting to be contacted by an intervention team shortly. I chased them up earlier today and spoke to someone there. Hopefully that will speed things up a bit.
I like the idea of not making a rash decision about longer sick leave when one doesn't feel quite up to it.
I have taken some other med (beta-blocker for agitation and anxiety) today so I could go through with the day and probably try doing this in the short term until more help becomes available.
It's great you manage to get on with things at work even if it's not easy, but at least you manage to keep doing things. I've always tried that and hope that I will be able to continue doing so. I'm taking it one day at a time now and hoping for the best.