I don't have a mentor and I never did. But I did get some advice from an intern during my third year of med school that haunted me for many years after:
When I was a third year med student on my medicine clerkship, a intern named Jim saw me working on the ward at around 6PM. "What are you still doing here?" he asked, shaking his head at me.
"Huh?" I said.
"You should go home and study," Jim told me. "Or else..."
Or else I'll fail my boards? Look bad in front of the attending? Flunk out of med school and end up homeless and penniless on the street??
"...Or else you'll end up in internal medicine."
"Oh," I said, confused by Jim's ominous tone of voice. "Actually, I want to do internal medicine."
"Oh god," Jim said.
Interns typically are the most miserable and bitter people you'll meet in the hospital, but Jim was especially miserable and bitter because he wanted to match in radiology but didn't. He was stuck doing a prelim year in medicine and wasn't having much luck finding a radiology program that would take him. One day he was complaining about it and said to me:
"You know, it sucks. All this work going through med school and you can't even do the field that you want to do."
Those words really haunted me. I eventually came to realize that internal medicine was not my first choice and when I matched in it two years later as a compromise (long geography-related story), I kept thinking to myself, All this work going through med school and I can't even do the field that I want to do. When I hid in the call room during my dreaded internship ICU rotation, I angrily thought to myself, All this work going through med school and I can't even do the field that I want to do.
So I quit. To do the field that I wanted to do.