If you feel stressed, you may be giving too much of yourself. If you are giving more then you like to, to compensate for your teammate, you might need to find teammates that are up to your level, that carr as much as you, an a business that care as much as you too.
But I think it's possible to continue to care while not letting the organization's sluggishness affect you in a personal way. You care because you believe it's the right thing to do and/or because it furthers your skills. Good. The others are missing out; even if their pay is the same now, in 5-10 years you will be better positionned than them for whatever opportunities come. At the same time—and this is where that balance comes in—if you're slaving away with no enjoyment of life, it all seems pointless.
I've seen a lot of people fall into that trap and then get bitter when the "less productive" person was promoted.
Of course sometimes the meta is licking your bosses boots not something that makes the company money...
Are you measuring feature completion or business impact? If your managers are happy with your coworkers that “produce less”, then chances are their efforts have the same business impact as yours.
It's a project with a fixed span, and the barrier to entry is very high (government clearance and certain skills) so many people who gain entry exploit the situation because they know we're short on people and won't fire anybody unless they have a negative impact on the business. So the bar is high to get in, but the bar is very low to not be kicked out (have some positive impact). So we'll keep people that only do 1-2 tasks per week because it's better than 0, but don't reward the folks who can do 10 tasks per week.