I get it. It's a large, awkward instrument, and it's never going to be as easy to stretch your hands to reach those high notes as on the violin. I'm here to offer a tip, though, which for me has worked well:
Wrap your thumb around the shoulder of the viola.
If you're already doing this, then skip to the next paragraph. But note this advice does go against standard teaching, at least for the violin, where you are traditionally supposed to keep your thumb on the end of the neck of the violin, and not wrap it around any further. On violin, it's nice to strive for, but for viola, I think the teachers who parrot this rule are just privileged with big, flexible hands and relatively small violas.