Going to be really honest with you OP, and I will probably catch a lot of hate for this.
As much as people hype this career up to be a humanistic calling, and that you need to have a purpose for it, a passion, and whatever else people say you need to have to know this is the right career for you, I do not feel that those things are necessarily are true.
At the end of the day, this is a career that people choose for a lot of different reasons. Stability, pride, money, patients, experiences, etc, etc. Now, to be honest, this is very hard. There are a lot of really rough days, a lot of "WTF am I even doing here; all my friends have babies, wives, and houses." So, having passions, drives, "callings," makes it easier. It gives you something to look back on - or look forward to - to remind you why you are here, and what is keeping you going.
For me, it came down to coming from a poor, rural background and my mother having a chronic illness. Having the intelligence to do something no one else in my family could accomplish, the drive from my mother's illness, and the desire to change my family tree forever (aka get out of poverty), I chose medicine. But even now, I do not know if I "KNOW" I want to be a doctor. But I do know that being a doctor gives me all of the things I want out of a career, coupled with the fact that I get to change other peoples' lives for the better which is incredibly humbling.
Only you truly know why you want to do this, but I can tell you this. If you got into medical school, then you can get through medical school. You can do this. Its scary. I got accepted 6 days before school began, and had to move three states away in all of 2 days. Change is scary. Especially when you are looking down the barrel of a gun that very few people understand. But the people that are here, the people that have been through it, and will go through it with you, will always be there to help you. You can bank on that.