Another few Piano tech I need help with.

I need help with my thumbs "there long" not falling between the keys with it nail or hitting 2 notes because bent. 
I have done my scales slow and work my way up to about 100 in few minutes. So I know them I've also played Bach invention 1. 
People tell me I just need keep practicing but I sit for hrs practicing straight into Fur Elise than my compositions. 
And some times my hands are ice cold or very very slippery on the keys "digital piano" and this all frustrates me and I'm not getting much help.
 
Thanks

One thing my piano teacher emphasized was that I needed to play on my finger tips. I would be lazy and play with my hands level with the keys. She made me focus on having my hands above the keys over them.... like "walking a spider" with your fingers. That prevents the thumb from easily pressing 2 keys at the same time. BTW: She was a small lady with short stubby fingers and she managed to play some amazing things. As a young (early teen) my finger span was actually longer than hers.
 
She also tried to teach me to feel the music as I played it with fingerboard technique that resembled rolling waves..... hard to explain with words..... but after all these years, I finally "get it".