One of my quilting free motion teachers said that she always has 'cheater' quilts put together(they are for babies, or for a donation for a worthy cause. I would first try one of those. The other suggestion is to start the quilting session on a throw away, same idea, but it lets you start practicing first on an unimportant piece so you hopefully find your mistakes before you make them on the quilt you love. I also found that you should plan ahead the type of quilting you want on what part of the quilt. You may want to do a grid for one part, a meander around another, little e's or l's on a border, you get my drift. Anyway, you don't feel so panicked when you reach another area.
John Flynn takes his practice pieces and sews them together. He then uses a weed wacker on the seams to make a raggedy quilt. He shows himself doing it on his website.
Finally, when you start making mistakes, and you will, stop. Take a break. Walk around do something and come back to it. You will find you will work much better with the simple last suggestion.