Oh, my friend, Nurizeko...I feel for you, yes, I do !! There is a psychological term for that, and while it has slipped my memory at the second--and I'm too lazy to run over and trace that article down--let's just stick with the 'winter blues'.
The actual key to revitalizing the chemicals need to counter the present state, is, SUNSHINE !! Yes, more than anything else, that does it, according to the article (and that's Scientific American, we're talking about.)
One more thing you could do is synergize. Pull out the ole six-string, and get down heavy into the low down dirty Blues.....putting those two energies together will work to cancel out each other, and the end result is a postitive, up-beat, and sitting on the edge of spring emotional state.
Having a problem with a gf can be a problem. But, my friend, it's not as bad as having a problem with a ww...so do feel at ease...hee, hee, hee. I look forward to hearing of your improvement and smiling face !!
OH...yeah, 'ww' stands for Wrathful Wife.