I think the hardest thing for me, at this present time, is focusing on my style. There's so many different styles out there and so many amazing artists. When you see an amazing style you like there comes some time, some where, when you incorporate it into your own. I've probably done it many times as well as other artists. It's like an evolution thing. You never do it on purpose, but your eye was drawn to something in their pictures and instinctively some of those qualities make it into your own work.
Trying to stay away from the habit is tough. It makes you look at your art and think that it's nothing but a copy with nothing unique about it. Even if you try to stay away from the habit it still happens. I feel this is part of building style, but it shouldn't be the ONLY way you build your style. Experiment and find things you like.
I'm not encouraging being a recluse but shut yourself in when practicing without a reference. Try new things with different tools and different colors. Experiment with sizes and shapes. Scribble a bit and find something that attracts your eye.
I think the worst thing an artist can do to themselves is to look at their art and to find it unoriginal or feel that no one will have interest in it. Most of the time it isn't about what other people like about it, it's what you the artist likes about it. Your art is a piece of you and you should build it to express what you want to express. Even with this being the case always be sure to think about the criticism you get, good and bad because someone may point out something you yourself didn't notice. You may benefit from it in the end and come out a better artist for it.