Is it because you are on a green horse? I know i'd be more confident on a more experienced quieter horse myself.
Yes, I think that is the main reason...this horse gave me a very very hard time when I first started riding her. She would stop at almost every jump, and I would go flying off the front of her. I must have fallen off her about a million times. She is much better now, and only runs out when I get nervous and just sit there like a lump. And she doesn't stop anymore, just goes around the jump. Which is all my fault.
But I jumped her about 3 feet this weekend, and she was great, and I wasn't nervous at all. I just picture me going over the jump and was determined to get her over it, and she went right over the first time. I know if I keep having rides like that Ill get my confidence back.