These are timely questions, because a couple of my newer co-workers are going through the same issues, and I would really like to be able to help them along. I've seen them both--caring, hard-working nurses--shoot themselves in the foot through lack of confidence, and I can remember doing some of the same stupid things just a few years ago. Plus, I'm learning to do charge, now, so I'm facing some of the same challenges they are, in a way, except that I've already survived my first year as an RN, so confidence isn't as much a problem. (If you can get through that first year, you should pretty much be able to get through anything. I still sometimes find I'm not entirely sure what I'm doing, but it has been awhile since I've felt like I couldn't either figure it out or find someone who could. Knock on wood.)
I think the two biggest obstacles to critical thinking are panic and the simple, ordinary mechanics of the job. Back in clinicals, all those many months ago, when I passed a med, I could tell a patient what it was, why they needed it, what the side effects were, yada, yada, yada, then I'd struggle to get the package open. And then, maybe a fourth of the time, the patient would have been taking the med for something off-label for a couple of years or more, and they'd tell me why they were really taking it. Well, you know, that still happens, now and then. I'd been a nurse for a couple of years when I had a 10 yr old (or so) seizure patient getting clonidine. Uh-oh, look-alike, sound-alike!!! Paged the doc, pronto, and found out clonidine is sometimes prescribed for behavioral problems--ADD, if memory serves. Told the mom why the med was late and she laughed and explained it to me better than the doctor had. But, of course, I'd lost all hope of ever being Supernurse, by then, so it was just a learning experience, and not a crushing blow to my self-image.
So, anyway, I think a big part of critical thinking is just a matter of getting better at the nuts and bolts, so that you have the time and available brain cells to look at the big picture. I guess it's a lot the same with confidence, although I do like the saying "Fake it until you make it." Not meaning fumble along blindly, but there's a big difference between, "I'd better get some help!" and, "I think I'd like to get a second opinion, before we proceed..."
I don't know how well I'm explaining that. That's the big problem I'm finding with my new friends--it all makes sense, once you've done it, but try to put it into words and it's practically jibberish.
ETA: One of the things I used to tell myself regularly, and still do from time to time, is: If they wanted a good nurse, they could have hired one. It sounds terrible, and I really am grateful they were willing to take a chance on making a good nurse, rather than buying one ready-made. But I truly believe I took a lot of non-productive strain off my brain when I learned to aspire toward becoming marginally competent, rather than great. Mind you, I'm not giving up on great, but it ain't gonna happen overnight, or in four years. I'll be happy, now, if I can be a really good nurse while I'm still ambulatory.