Yeah, it's called Imposter Syndrome and it can be a problem if it's not taken seriously.
As for when I felt it going for me and when I felt I was cut out for it was frankly me just making the decision that I should go into it as narcissistic as that sounds.
My dad really took a lot of time teaching me programming, math, etc. but when I got to high school, there was a cute girl in my IB Bio class who I would impress by knowing random Bio stuff and I think I was just good at it because I was the only one who read the textbook instead of the IB study guide. Chemistry was also pretty interesting and Physics was the most interesting.
Growing up, I obviously saw my immigrant dad as a role model and he got where he wanted by picking something he wanted, committing to it and putting all his heart into it. Therefore, I'm not the traditional person who saw what a doctor did and was enamored by it. I looked at law, finance, engineering, and medicine and looked again seriously at Medicine and Engineering and decided on Medicine because I really liked teaching and giving advice and thought the subject matter was interesting and the pay was great.
In college, I went in knowing what I wanted,
and focused and shadowed not because I wanted to see if I liked Medicine, but to figure out what being a doctor required and so I could say I did it on AMCAS. Then I jumped through all the other hoops and worked my hardest on the MCAT/GPA. Everything hasn't been easy and I was miserable during periods of second year before Step 1 but I am in love with what I've seen so far going into 4th year and I am glad I had a decisive mindset early on.
There are occasional checks when I make stupid mistakes or but I think I'm pretty happy overall and I seem to fit well with the teams so I'm confident I belong.