No, you are not wrong to feel completely unappreciated. It sounds like you are a very dedicated employee and being taken advantage of as well. $1,500 is less than $32 gross, so yes, it is paltry. It's not unusual for those with the longest length of service to make less pay as salaries increase over time and salary ranges increase. It can be tough when new hires reap the benefit. Correlating the pay increases to billable client hours, sounds wrong to me. And, I know it can be very irritating when a coworker does not have the same work ethic, and frustrating when management doesn’t recognize it, or chooses to ignore it.
With all that being said, I guess my question is, what do you want to do about it? Your attempt to get more money was fruitless, but you may want to try again. Of course, you can't give an ultimatum unless you are willing to leave, but that is an option.
You aren't hourly, but at some point, working 7 days straight and long hours mean you are basically working for free, or to no benefit to yourself. Employers will keep working you as much as they can get away with.
If you don't want to leave, your only option may be to change how hard and how much you work. If others seem to have a life, and can go to lunch or leave at a reasonable time, maybe it's time for you to woman up and do the same. Make some hard and fast rules for yourself and stick to them. Start saying you are unavailable due to another commitment or appointment. You don't need to say what it is, it could be getting home at a decent time. I have a couple of blog posts on this that may be helpful.