Would I love to work this on my deathbed?

This might seem a bit extreme criteria to evaluate what you really want to work on but answer to this will tell you whether the job that you are doing now is meant for you or not. Whatever you would love to work on your deathbed you should be doing that now. Earning livelihood by doing some job will make you like so many people who want to just do their job, earn, retire early so that they can enjoy their life -- metaphorically it's like saving up sex for old age. Why would you want to retire if you love what you are doing? If you don’t love your work then nothing great will ever be created by you.

There are hordes of stories of people working unbelievable number of hours everyday, people working very hard despite very unfavorable conditions. You can't make those people work like that just for livelihood, they work for their love. People working on job wait for 5 pm.

Mathematician Euler lost one of his eyes in his twenties but that didn't handicap his love for math. In fact he went on to say that "I’ll have less of distraction". He kept working and lost vision in his second eye at age of 70. Even complete blindness couldn't stop him working on solving mathematical problems. He proved to be even more productive in his blindness and worked for 17 years in total blindness.

This is not madness for mathematics. This is epitome of love for the job/love.

Find your love.  If you haven't found it yet, keep looking. Don't settle.