So, an update on this - I went to the doctor on Tuesday and she told me that, just like asthma or something, depression can get worse for no apparent reason from time to time, and said that I should double my dose to 40mg and go back in a couple of weeks to confirm that it is working.
So my next question is... how long might I expect to wait before I can tell if it is working or not? It has been 5 days today and it definitely hasn't had any effect yet, so of course my messed up thoughts try and tell me that means it will never work at all and everything is hopeless. I know that's rubbish, but still...