The Internet is a strange place full of people who will tear you down whenever they can despite lacking the requisite qualifications and experience to do so and the tact to do it respectfully. Here's how to handle those who seem like they just want to ruin your day.
Coming to you from Sean Tucker, this great video essay examines the unfortunately common phenomenon of Internet trolls. Among the many insightful things he says, Tucker is particularly right about the lack of talented trolls out there. By and large, the vast majority of trolls are faceless or unaccomplished in their field, and when you call them on that, they'll offer you some sort of faux logic about how their identity and accomplishments don't matter to their critique, except they do. Just like you wouldn't and shouldn't listen to medical advice doled out by any faceless self-proclaimed expert, nor should you take that of a self-proclaimed photography expert. In fact, in all my time and involvement in this industry, I can only name one marginally talented troll I've come across, and his problem was more raging ego than insecurity. Really, the battle against trolls comes down to you: know who's worth listening to and don't waste time or energy on those who aren't.