I think I know exactly how you feel. For me, I decided when I was 9. Ever since then it's been the easy reply whenever anyone asked me what I wanted to be when I grew up: "I'm going to be a vet!" Plus my mom for the past several years has been making the "I think that's awesome; you decided what you wanted to be at such a young age and you've never looked back!" comments. I guess there's a part of me that thinks perhaps I should have looked back. I honestly don't think I would have changed my mind had I given everything a "second thought," but there's a part of me who wonders if I never "looked back" because it became so comofortable to have an answer for what I was going to do with my life.
Plus, there's another (LARGE!) part of me that's absolutely afraid of failure. And now someone's saying "here you go! you're dream is coming true! you say you wanna be a vet, so we're gonna invest time and energy teaching and training you" and I'm like, what if I screw up? What if I go through vet school and SUCK at being a vet? Then I'll have disappointed the school that agreed to teach me. I'll have disappointed my mother who "thinks this is so great!" And, most importantly, I'll have disappointed myself and I'll have made these past however many years of working at vet's offices, doing extracurricular activities so I can be "well rounded" etc. etc. worthless. (Of course, even if I'm not cut out to be a vet I know I've loved those experiences anyways, so of course they're not worthless!)
So this is what I've done. I've told myself, no matter what (unless I seriously want to kill myself), I'm going to get through vet school. And, I'm going to work in the field for a year. At that point, if I don't enjoy it, I've promised myself I'll go do something else. I won't regret vet school, I won't regret all the hard work, I'll try not to regret the thousands upon thousands of dollars in debt I'll be, and I'll go do something else. Assuming I don't specialize etc., I'll be about 27 or 28, which is definitely young enough to "start over". And the thing is, just telling myself that and taking the pressure off "I have to decide right now if I don't want to do this!!!!" has made me a LOT more excited for vet school to start again. So my recommendation? Make the same promise to yourself -- and MEAN IT. You don't want to let your fear of failure or whatever it might be hold you back from doing something you'll love, but at the same time you don't want to let your fear of regretting vet school make you keep a job you genuinely don't like. So try it out and see what happens. And know that you're not alone; there's more than a few of us who are hoping our feet warm up soon!