Whither imagination?

A strange observation perhaps, but it appears that I have lost my imagination. Not completely, of course; I can't imagine, if you'll excuse the pun, what that would be like. But what's gone is my creativity. I used to write a lot. I'm not saying any of it was necessarily any good, but people seemed to enjoy it. I've actually written two novel-length stories. Most of it was humour, some of it was black humour, and I really enjoyed writing it. I would write when commuting, I would write on holiday - whenever. But now I just can't get started. I don't think it's writers block, because it's not like I'm stuck on any one thing, it's just if I try to start writing something, I just think it's rubbish or disinteresting or I can't get into the flow. It truly is most infuriating. I also think it's probably the one thing (apart from not being able to drive) that gets me down about my condition. All the other stuff that people think I should get upset or depressed about with regards to having a brain tumour don't seem to bother me in the slightest. But not being able to write is a real bummer. I'm just glad that my income doesn't depend on it.