My best time is 26 seconds, and I've gotten that both with Friedrich and with Petrus. I now exclusively use Petrus because I know all the PLL algorithms but I'd rather just know 7 OLLs than 54 or however many there are for Friedrich. I've plateaued at a 40 second avg for what seems like years. That was true when I got good with the beginner's method, it was true when I learned Friedrich, and now it's true after I've learned Petrus. Am I just a terrible person? Is this the result of my sins coming back to haunt me? Is there anything I can do other than just keep solving it constantly (which is what I'm doing now)?

before I can answer your question I need to ask a few of my own. Is your move count reasonable? How is your turn speed? Do you look ahead well? About how long do the individual steps take? Is your cube any good? Can we get a solve video? That's about all.

Edit: Oh, and it's called the fridrich method, not friedrich.

I use a Dayan Zhanchi. The only time I'm consciously aware of looking ahead is when I'm solving the cross. I'm sure I have basic abilities in looking ahead but I can't specifically point them out. This first video was me trying to do the Petrus method but I freaked out and went to Fridrich. Solved in 31 seconds. The second video I used Petrus and it took 36 seconds.
http://www.youtube.com/watch?v=fGniRLViAAY
http://www.youtube.com/watch?v=f6uJ5GX2DIo

I should mention that I only learned Petrus method last week.

One thing that will really help your solves to go faster is to slow down your turn speed. believe it or not, you have a very fast turn speed for a 40 second solver. you are turning so fast that you cannot look ahead to see the next pieces to solve. try turning smoothly, and consciously think about looking for the next piece to solve. One great thing about having a fast turn speed is that you can do your last layer as fast as you want. give it your all on those algs! good luck, and it's good to here that you're practicing.

I agree with this. I haven't practiced in a year or so, but I used to average around 30 seconds and my turning speed was less than half of yours. Watch the first video again, and pay attention to how much time you spend just staring at the cube. In the first ten seconds after you start the timer, you did almost nothing, and from there out there continue to be multiple long pauses. Decreasing those are where better times are going to come in.