Can I chime in here?
I'm a third year med student, and I can say without question that you never know for sure. You constantly question whether you can hack it, whether you want this lifestyle, whether it's worth it, and whether you made the right decision. This is healthy. You should question it all along the way. It keeps your priorities and conscience centered on justifying yourself in the face of unavoidable uncertainty. Questioning "is this worth it" forces you to answer that question for yourself and justify your commitment. There's nothing wrong with reinforcing your commitment in this way. Those individuals who say, "I knew when I was 4!" or "it's in my blood" or "I have a disease that confirms it for me" are probably deluding themselves to a certain extent that they're any more sure of their decision than anyone else. They may have a perspective that offers a unique set of priorities (thus making the answer to the "is this worth it to me" quicker), but they don't know any better than Random Pre-med Joe does.
The only way to know for sure is to do it. If you're compelled strongly enough by passion for what you've seen so far to apply (and apply with as much of yourself as you can) and you get in, that means that your passion for medicine and your experience in medicine has enabled you to follow this path. Take that as a sign that you can and should do it. There are a few individuals who get in and realize it's a mistake....and there are a few individuals who don't get in and really are fit for medicine, but these are pretty rare. The point is, the only way to prove you're ready, willing, and able to be a doctor is to walk the path. There's no DM-I or shadowing or research pre-med experience that confirms it for anyone. You'll find that even talking to attendings...they still question it. They question whether they're in the right field, whether it's all worth it, whether they should change what they do.
Questioning makes our world go around. If we didn't ask questions, we'd never advance medical science. We'd miss countless diagnoses on patients. We'd end up with regret at the end of the day, because we never took the time to brood over why we're working so hard in this pursuit to be a good doctor. It's not obvious and it's not a given. Lastly, it's not for everyone. Take what passion you've got in you and follow it with everything you've got in you, because that's the only way you'll live a life without regret.