I was talking to someone a few days ago about happiness and what it would take to be happy. She said money would solve all her problems and I partially agree, being "comfortable" financially would definitely take some stress away, but I've never been one to put much value into money.
Sure, it's nice to have and be able to buy food and treat yourself and do things that are fun, but for me, even when I have absolutely no money I still feel pretty content.
I don't think happiness can be a constant, much like depression and sadness, that too passes. Rather, I think they're both extreme ends of the emotional scale, feeling neutral for me is the default. Occasionally, you'll have a trough of sadness after the loss of a pet, loved one, a breakup, maybe your favourite series gets cancelled. Then other times, you'll have a peak of happiness, a new relationship, engagement, winning a game of rock paper scissors.
Emotions are up and down like the waves on the sea, you can have the peaks and the troughs but the level of the water remains the same. (Lets assume tides don't exist otherwise this already bad analogy becomes ridiculously bad)
I think my point is, don't let the sadness/depression in your life stop you, because it will pass. Equally, hold onto those moments where you feel happy, cherish them, because much like depression, that too is likely to pass until the next peak.