Do some weekly volunteer work to help others, even (or especially) if the work is "beneath you." It's not a guaranteed cure-all, but it's highly likely to make you feel better -- it gives you a different perspective, and reminds you that there are people worse off than you (and there seems to be evidence that humans like to feel that they're better off than others).
(Some evolutionary psychologists also think there's evidence that "feeling good from doing good" is a selected-for trait. That's because the doing-good that results from the feeling-good can confer advantages on kin or on a group. EXAMPLE: Other things being equal, a parent who happens to derive pleasure from caring for his/her offspring is more likely to have his/her genes passed on to grandchildren. EXAMPLE: OTBE, an infantry squad whose members happen to feel an urge to look out for one another is more likely to succeed in war than an every-man-for-himself mob.)
(Some evolutionary psychologists also think there's evidence that "feeling good from doing good" is a selected-for trait. That's because the doing-good that results from the feeling-good can confer advantages on kin or on a group. EXAMPLE: Other things being equal, a parent who happens to derive pleasure from caring for his/her offspring is more likely to have his/her genes passed on to grandchildren. EXAMPLE: OTBE, an infantry squad whose members happen to feel an urge to look out for one another is more likely to succeed in war than an every-man-for-himself mob.)