Don't worry - like almost every sport/skill one goes through a phase of thinking that you've cracked it and then it all goes to worms for a bit. Golf is a classic example (so I'm told!)
With gliding it's the same - you crack the operation of the stick. Then that difficult b******* in the back makes you start using the rudder & the stick control goes haywire for a while & then it all comes together. I expect you've done that bit.
Then when you've cracked landings with him doing the brakes, he makes you do the brakes and it all goes to pot again (but only for a while). So don't take it to heart, have fun.
Swapping from glass to K13 & back does not strike me as beneficial though. Try to stick with one or the other if possible.