Maybe you're tired... you should stop before you start to get tired and unfocused. End on a good note. I don't think it's really a mental block.. just if you do the same thing over and over or if you do different things for a long while, then you get tired and your brain gets fried. Thus you ride poorly and the horse doesn't give forth any effort. You both just turn into lazy buckets.
Does that make sense?
Go back to basics.. don't drill yourself on them. But do some good basics, and then try some of the new stuff or stuff that you have some trouble with. Try explaining things to yourself. "I'm a fly on his back" therefore you go WITH the movement.. you push forward.. a fly can't hold back a 1000lb animal. Maybe that'll help
Good luck :) You're doing great!
"Strength is the ability to use a muscle without tension"