Milzy...i don' know if this relates to how you're feeling but ...
I have a moderately important job in the sense my work keeps the factory running and several hundred people being productive and the company being profitable if my work is done properly and diligently. I love my job but hate the politics, the 'it's not my problem' mentality, the constant pushing of workload down the food chain, and so on and so on...i just want to do my job well, without constant distraction and interference...and it bugs the hell out of me sometimes, the politics etc, i can get really angry and peed off sometimes, to the point i want to look for another job. The people are nice but the system stinks.
But then, occasionally, i have a what I call, a mental reset. Fook the lot of em, say yes to the bothersome stuff, smile and either do it, or say yes and send them away happy...and don't do it. The system isn't going to change for me, it's me that has to fit in....or ship out.
It' very cathartic, you can change your outlook at work with a change of your own expectation. When i do this i usually feel better almost instantly, sometimes after months of getting more and more peed off.
Theres' also the impact your poor mood has on how your colleagues view you. I'm sure I'm not very good to be around when my mood is low, conversely, when my moods good and I smile...people smile with you, you become a better person to be around.
In simple terms, it's up to you to adapt. It can be very very effective and change the way you feel immensey.
Hope something works for you, there's nothing worse than feeling like utter crud at work.