I, too, used to have an aversion to large wounds, blood, etc. As a matter of fact, I can remember clearly when my teacher was speaking about TPN and a diagram she put up showing the subclavian vein, and I thought I was going to pass out right there in class. This phobia almost stopped me from applying for the nursing program, since it was so bad at one point, that I could not picture myself ever being able to hang blood, or change a wound dressing.
However, what I did is desensitize myself by watching any and all medical procedures on the health channel, purposelly finding pictures of same online and just exposing myself as much as possible. I also got a student nurse job at a hospital where I was sure to see and deal with, in person, such things. I always made sure to eat (before going to OR or doing anything that I knew would make me feel like fainting), and now, I am packing deep abdomen wounds, hanging and cleaning up blood without a second thought. As a matter of fact, I am now so desensitized, I have accepted a new grad position in a level II ED.....something that I would have never thought I would be able to do.
Hang in there, and try to expose yourself as much as possible to what it is that makes you feel sick, dizzy or whatever because eventually, you should become so desensitized to it, you won't be bothered or at least not to the point that you are unable to deal with it.
Good luck!! I think you will do fine.