It's a very strange thing to be the person on one end of the lesson watching the other person work at something, accomplish it, and then… say they can't do it. I've actually seen this happen before my wedding hiatus, but I noticed it far more after I'd been away for a while. It has BOGGLED my mind for a long time, and I finally made a breakthrough last week!
Turns out, my students are such go-getters that they focus too much on what they can't do, versus what they just did. The weirdest part is that they all have very positive goals for their practicing, but when they are actually in the act of practicing, all these negative thoughts are getting in the way. It's very strange to be so consciously in control of your thoughts to steer them in a more positive direction, but let me tell you… the difference in their playing was immediate and huge!
So guys… yes, work hard, but don't beat yourself up. Beating yourself doesn't do anything to build yourself up to be the best that you can be--it takes you further away from that. Build yourself up with positive reinforcement. You've got this. :)