One thing about going back to school when you're older is that you're more relaxed about grades. I stepped off the "must get perfect grades" treadmill when I graduated from college, and when I went back to grad school, grades just didn't stress me out as much.
This was a good thing, because I was taking a couple courses that I didn't have all the prerequisites for. That's one problem with going to a SLAC for college - a lot of my physical science courses were just general chem, physics, bio, whatever. My grad school was a big research university, so if you were doing, say, contaminant related stuff, you'd do contaminant chem, physics, bio, which diverges from the "general" curriculum early on. So I had some pretty big holes in my education. I was determined and fairly confident, so I thought I could just pick things up as I went along. And I did.
The problem came in when I was expected to figure out a whole bunch of complicated stuff in a short midterm. I hadn't taken a test in years, and certainly not one that I had no hope of finishing in time. I sucked.
Luckily, lots of other people sucked, too. But mostly not as spectacularly as me. The prof curved the exam and, well, I still didn't pass. If this had happened in undergrad, I probably would have a) cried, b) asked everybody except the professor what I should do, and c) developed an ulcer. I'm proud to report that I did none of these things. Well, I did get a little choked up at one point, but we'll ignore that.
What I did was meet with the professor. I said, "I do understand the material. But I didn't have time to go through even half of that test. I was working through stuff that I needed to have essentially memorized. Is there any way I can salvage this course, or should I drop it now?"
And I did salvage the course - I worked my ass off on all of the assignments, spent more time studying for the final than probably all the finals I'd studied for in undergrad, and pulled out an ok grade. It helped that the pace of the 2nd half of the course slowed way down for me - we had flown through all this stuff that was review for everybody else, and so the "new" stuff was a lot easier for me to keep up with.
My cumulative GPA in grad school was essentially the same as for undergrad. But my stress over grades was nonexistent in comparison. Why? Because I had finally internalized that I was in school to learn, not to get specific grades.