How about a gratitude journal to start developing a positive outlook?
There is Gretchen Rubin’s Happiness Project - a lot of stuff free on the web, including YouTube videos
Negative emotions are kind of self-fulfilling, and feed on themselves to grow.
What sort of things do you complain about? Workload, type of work, relationship with boss, peers, lack of time, lack of recognition, lack of personal time, lack of emotional support from friends, family, partner, lack of respect....?
What time of day do you typical complain the most?
Odds are you don’t complain about EVERYTHING (if you do, I loved you on Seinfeld) - but if you can start seeing patterns it can help to get more specific suggestions on things to try.