Good to hear from you Isychos, and happy twentieth to you too. I bet several of our regulars will have good comments on this.
I would say that this is the key: "Am I applying the measurement of pleasure vs pain correctly within the context of work?"
In asking that, it seems to me you are maybe expecting every aspect of your life to have more pleasure than pain, and probably it would be more consistent to be looking at the overall total and result rather than thinking that you (or anyone) can be successful in creating more pleasure than pain in *every* aspect of life. No doubt there are parts of life that are very difficult to be made pleasurable.
But work of course is a huge part of most peoples' lives, and if your entire work life is dominated by pain, then it is hard to think that in most cases someone would be successful in having total pleasure win out over total pain if you devote so much of your life to something you find painful, especially if "the mental pain extends outside the working hours, which is very draining and does not allow me the energy to discover and devote to pleasures in my personal time."
In my own case it always helps me to remember how short life is, and that despite whatever duties or obligations I have taken on, I only have a relatively small amount of time to accomplish in my life what I would like to accomplish. That helps me prioritize my time and gives me motivation to put aside projects that aren't as productive as they should be.
But another and maybe more fundamental aspect is that no one can tell you but you how to get the most out of your life. No doubt people can share hints and their own experiences, but your feelings are your own and you have to work with them to decide how in your own specific case you will best maximize your pleasure and minimize your pain.
These are only brief thoughts but I am sure most of us have similar issues so the more you comment on your questions the easier it will be for others to respond.