You need to figure out what it is that you are afriad of, or is making you nervous, or is even making you feel you are incompetent. Afraid of falling? You will have to get over that one dear :) Does it make you uneasy when others watch? Then get a private lesson or do it when less people are at the barn. Is it that he is freaking huge and has a huge gait that is different? Yes, that is quite a difference but it should not make you feel bad. I know this feeling you are describing, it has happened to be before when I got on a 17.1 jumper that was heavy on the forehand and all around scary. You will get used to this horse. Don't be afraid to fall off because we all do and if you can manage to do it with grace when the time comes, you may be able to get up and get back on! Don't be afraid to get off balance at the trot if it's large, or coming out of a fence, because you know that you can just slow down and recollect and get at it again.