How much time do you have to decide? I think shadowing a nurse for a while is a great idea. It is a tough decision to make, and from what I've heard, the schooling itself as well as the job can be very stressful. Maybe it's not for you. Don't feel bad if you decide not to. It's your life, and you should be happy with whatever you decide.
Frankly, if you WEREN'T excited to get the letter, than maybe you've already answered your own question. I know that I dreaded NOT getting a thick letter, because I knew what the thin one meant. If you can't muster up any sort of excitment for it, then maybe the spot should go to someone that really, really knows this is what they want. I don't want to make you feel badly at all, but if you're this apprehensive right now, it may not be for you.
I wish for you peace in deciding what you really, really want. Life is too short not to be happy with what you are doing!