In the Tools section of this site there is a tool called Monitor Your Arousal Level which I sometimes assign clients who've experienced continuing anxiety, stress, or worry at levels they want to reduce significantly or eliminate.
The same tool also works well in reducing sadness, dejectedness, and feeling down. For that use I'd call it Monitor Your Mood Level. And, I'd ask that you think of a continuum between feeling really happy and feeling really "down," with zero being down and ecstasy being ten.
I'd ask you what number range you'd like to be in most of the time, maybe somewhere between six and nine.
Then I'd ask you to, several times a day, whenever you think if it, ask yourself what number your mood/emotional state is at that moment. If it falls below your preferred range, you are to use one or more of the tools in the tool section (or in this series of posts) to raise it back up into the preferred number range.
What you'll find is that the first few days you'll be doing a fair amount of tool usage to keep bringing your mood back up to where you want it to be, but that gradually it will be less and less necessary to do it. And, interestingly, you'll discover at some point that you have, from time to time, automatically raised your mood level without even thinking about it.
This technique is one of the most useful techniques I know to make ecological and lasting changes in how one feels.