Years ago someone here asked about mapping their own worlds. Most of us have one in the back of our mind's eye. Maybe it's the game world we use for an RPG with friends, or maybe it's a world we write novels about. The interesting thing was that even our best artists here were saying things like, "I don't think I'll ever map my main world". The reason given was they felt like they could never do it justice. I have this problem myself now, I did try it once a while back and I keep meaning to go redo it but well, it will never be as good as I can visualize it in my mind. It sounds like this is what you are bumping up against. What works well for that is to just do as others have mentioned and go make some maps that are not a part of your world. You'll get skills from that and you can be free to explore odd ways of doing things which will free up your creativity.
All artists should be able to look at work they have done and know they can do better next time. That just means you are improving.
If you try to copy someone else's style you will never be as good as they are at it and you will always feel like you don't measure up. Find what you do that's different and embrace it.
Someone said, "let your art determine where you go with it" rather than trying to control it...or something along those lines.
I know I can get frustrated when I can't artistically convey an idea I have because for whatever reason my skill set isn't good enough. That just means I'll have to keep working to improve.
Here's the view of one of the leading lights of illustration on just this question:
Muddy Colors: 10 Things...Word List