Horse Jumping Help Needed?

Ive been jumping for a few month now and the highest I've jumped is 1metre (3ft) I find that when I jump I feel very unbalanced and always seem to get flung out of the saddle a lot. Also I fall on the horses neck just when weve landed. This only seems to occure on jumps approxamatly 55cm+ . Can someone please tell me what I'm doing wrong and how I can have a balanced jump and feel safe in the saddle while jumping? Thank you :)