You are underpaid. It is true! And that is a good thing.
A couple of Sundays ago, the sermon was on service to God. The parable of the master that gave his servants money
to care for was the topic of discussion.
The master was well pleased with his servants that made him money, but was not pleased with the one that give the master back what he was given.
Let's extrapolate that to a more modern concept: your employer and you.
Basically, your employer hired you to make money for him (in modern paradigm, the stockholders). You were hired because it was perceived that you would be adding value to the company. Now, if you are getting paid exactly for what you produce, then what profit has the company made? I hear the complaints about being not paid what one is worth often enough. That, from a stance of good business practice, must necessarily be true. If you aren't turning a profit, then what is the point of employing you?
The trick is to get the company to accept that you are worth more than what they are paying you, whether you dig ditches or are a CEO. A salary negotiation is nothing more than you convincing the company to accept a lower profit margin or you demonstrating that you are worth more than they first thought.
Now, get back to work!