I am nervous for some reason?

I don't know why! I am REALLY scared because last time I got nervous was about 2 months ago, right before a group riding lesson and then I got bucked off a 16.3hh heavyweight cob and hurt myself and it STILL hurts! (I landed on my back dogey and ripped a muscle. The time before, about 5 months ago I went riding and had my first canter and I fell off and jared my arm, neck and cut all my waist. It is like I know or something? Every day before riding, when I get nervous I fall and hurt myself. Some lessons I don't get nervous and I fall off and hurt myself. And recently (the last few times I have been riding my horse) I have been really nervous! And each time I think I will fall off and hurt myself! But i don't? Could it be leading up to a big fall? Don't say it is my horse that makes me nervous, because he is very good to ride (besides his stiffness) I am worried. I am nervous now! And I am riding my horse in about 2 hours time. We are riding in the big field today instead of the menage sized field on a slight slope. x