I have a real problem manipulating my left hand on the neck. I always end of clutching the instrument which makes movement on the E string problematic. What I don't understand is how you're supposed to hold the instrument so that you can press down effectively on the string without it slipping. Just about where on the hand should the neck rest? On the thumb? Or in the curve of the hand?
Now I am still a newb! However, taking lessons and the very first bit of business my instructor went over with me was having my shoulder rest for my violin adjusted so that I could comfortably support the violin with my chin and shoulder. This helped with proper left hand placement afterwards because I was no longer supporting the instrument with my left hand.
The violin rests in my left hand between the fleshy bit of my thumb and the base joint of my pointer finger leaving a nice empty space down were the webbing of the thumb is (if that makes any since?). Main thing is being able to support the violin fully with your shoulder and chin that way you are not grasping the neck or collapsing your wrist towards you to support the instrument.
This is a googled page that shows when you scroll down what I mean with holding the violin between the fleshy part of the thumb and the base knuckle of the pointer.