The voice can work hard for you when you need it to. When you get on stage, you trust it to get you through your performance. Before you can trust it, it first must trust you.
There are many ways that you gain this trust, but here we'll talk about how you gain the trust through how you learn and how you practice.
Imagine you're teaching a game to a child. He or she will learn faster, and embrace the newness of it all if you give them some freedom to explore in a playful way. They'll get things wrong in the beginning but they'll stay with it if they're having fun and believe that it will get easier. If you think that the child should be taking the learning seriously and try not to make mistakes and not laugh and have fun doing it, the child will be come frustrated, discouraged by their mistakes and soon give up.
This works with you and your voice. If you think learning a new exercise is work and if you take it too seriously, you will get frustrated and discouraged by your mistakes and give up quickly.
Whatever exercise you're working on, it must be something you can do. If it's not, then you need to back off to something easier. Something that maybe challenges your voice, but not so difficult that you can't figure out the right way pretty easily. A good way to gauge this is to ask yourself, "am I having fun with this exercise?"
Once you can do the exercise easily, start to play with it. For example, hold some notes longer, sing some parts faster. What I mean is, try to improvise with it, but in a way that you're staying with the original concept of the exercise. Once you've found all the ways you can have fun with it, you've mastered it.
Once you've mastered it, you're ready for something harder. And the big fun here is that the harder exercise will not seem quite so difficult and that's because you're ready for it. It will just be a bit more challenging but you'll soon master that one too. And on it goes...