For me, it was mostly input from people I didn't know.
I was always a bit unbelieving of friends and family who told me my readings were on target, because I figured they didn't want to hurt my feelings, especially about something with which I was clearly so engaged.
When I started doing online one-card readings for the Free Tarot Network, I began to believe the feedback, because it was from people who had no interest in making me feel good. That gave me the courage to offer more complex readings here on the Reading Exchange. Getting positive feedback from other readers here finally convinced me I was good to go. (And the unforeseen upside to that for my current practice is that I'm not taken aback when I am asked to read by other Tarotists, which happens a lot since my venue is a metaphysical shop.)
But I have never totally lost the feeling of "What if I'm not really any good?" I have, however, learned to ignore it.
I would add, also, that if people you have read for want to come back for more readings, it's a very good sign that you are doing just fine.