I love the story of when Mary was called to be a the mother of God. She answers with a full hearted yes-without fully knowing what that meant. Then from the moment she treasured everything she heard in her heart. She knew something special was taking place-but didn't understand how special it was. Now Mary started off by saying, "How can this be, I am a virgin" She is putting a limitation on God. We cannot put human limitations on God.
We have to let God work. We put these limitations on God, and when we do that, we find it hard to believe and find it hard to accept. So we have to remember that God isn't just entirely human-he is also fully divine.
I like to think that my answer as been a yes. I will say 95% of the time it is. (Yes to the Priesthood) But like it is said in the bible, " Even Gold is tested in Fire" So I and you are tested in fire- and sad to say, but I haven't always been able to withstand the fire. "(again, this is a different story) Nobody goes through it easy. A vocation though, does require a response to God, like Mary. Saying yes, and letting God take the wheel.