Here's what works for me:
1. Never work more than 40 hrs a week. I actively look for jobs that are laid back yet challenging enough.
2. Spend a ton of time with my dog: This might not work for you if you aren't an animal person, but just look at them, they do mundane things and they are perfectly happy with their lives. It is very inspiring.
3. Memento mori - Recently I had to deal with a health issue which can significantly alter my quality (or quantity) of life. I am still discovering what this disease is, but ever since I found that I have doubled down on enjoying life. Never ever underestimate the importance of having fun (Watch Randy Pausch's last lecture).
4. Self esteem : Always remember nobody is perfect. There are people who work their entire life for minimum wage jobs and yet don't complain about life. I would highly recommend volunteering - I have spent hours of my time serving meals to homeless people and just talking to them, it gives me a different perspective in life. As low as you might feel, you are probably still doing better (financially) than 90% of the world. Also, don't try to be your smartest self, try to be your kindest self. Being smart is overrated and being kind is highly underrated.