Relaxing hands after leaps and stretches

Hi! 

I am really struggling with this one... although I am getting better. I am currently approaching this very methodically, think, ATTACK, relax...  repeat ad nauseam... it's a slow and arduous process... but it works to a degree.

I have been studying with my current teacher for a year, who does wonders with my anxiety, but I still struggle.

I have been playing since I was 5 with a few breaks. I am 51 now, and have a degree in music. I also teach piano to 19 wonderful students.  BUT my inability to relax and my anxiety really hinder my progress with more advanced repertoire, especially when it comes to playing faster pieces. 

I currently am working on Schubert's Impromptu #3 Op 142. The first theme has some awkward fingerings that have odd stretches. I am practicing nice and slow trying to relax in between, while maintaining legato in the voicing. But once I try to accelerate, my hands tighten up and my shoulders go up. Ugh!

Any advice? :-)

Please dont say eat bananas... I am not a fan! LOL

THANKS!
Denise