The biggest mistake I made so far was, just buying stocks pretty randomly. While I didn't invest too much money, a couple of them were just going down (Blue Apron, GoPro). I pulled out of Blue Apron, because after I did a little bit of research, I realized that company is just not profitable. GoPro, I've been trying to hold on to that, but I'm a little worried it's not going to bounce back.
My biggest fear is, when I do build up enough cash to invest properly to make some real money, I make a mistake and lose the majority of it, if not all of it.