Do what you can; leave the rest to God.
There are so many things in my life that I have no complete control over — all I can do is give it my best. For these things, there’s no point worrying about what happens past that point. Just hope for the best, and know that no matter what, the best will happen.
If what happens isn’t what I wanted, then maybe that’s because I’m short-sighted and can’t see the big picture. In the long term, I must have faith that it will ultimately be what’s best for me.
TL;DR: It’s a subtle balance — knowing when to give it 100%, and when to leave it in the hands of God.