Hey Emily -
Take heart - it sounds like you have a similar attitude as I have had (and had to adjust)....."Sheesh, I have been riding a mountain bike for X months and I should be more of an expert by now." I think that I would get frustrated because I was riding with people who made it look so easy and I felt like I was barely making it! Especially if they started riding around the same time or a little earlier. I just had to realize that there was a lot to learn and some people have things in their background that help them to pick it up quicker.....Don't get discouraged, as it will get better. Doing things like checking your seat, air pressure, learning little tricks like getting back....everyone is different! The other thing that I have started doing (this is the end of my second year on a mtn bike) is that I have set up a little obstacle track in my yard....things to practice on. It helps me figure out where I need to be on my bike when confronted with a rock, a root, a downhill, a tight corner, instead of looking at someone else. And just as I have come to love hills on my road bike, I have come to love roots on my mountain bike.....if it doesn't kill you, it makes you stronger!
The best rides are the ones where you bite off much more than you can chew--and live through it.