I just watched a Science Presentation the other day and it talked about how stress inhibits the growth of anything in us. Once stress becomes a factor; your body and its cells go into "Protection mode" which completely halts any progress or adaptation. This goes for everything in your body, as well as reality. If an activity is stressing you out, you won't make any progress. This is why you pick up things and learn alot faster when something isn't stressful. The presentation continued on to talk about how perception changes our genes and also that it rewrites our genes. It's some pretty detailed stuff. Here's a link to the video if anyone wants to watch it: