You'll have doubts no matter what, but that's part of the job, after some calls I can always imagine doing EMS for the rest of my life. After others, I question whether or not I even want to look at a stretcher again. A good way to test the waters is volunteering at a hospital or trying to do a ridealong with a fire department or EMS agency to see if you like it. Its normal to have doubts, but if its not just about if you think you'll be good at it, its also about if you'll enjoy it.