I don't think you made a mistake.
The D5100 is not a bad camera body at all, and it offers all the basic tools you need to develop your skills. The best thing to do at this point is to get another lens—either a telephoto zoom like the 55-300mm or a fast prime like the 50mm f/1.8G—and work on your photographic technique. There are lots of tutorials online and in print. I would personally recommend Ben Long's Complete Digital Photography; online, Cambridge in Colour has some of the best tutorials available, though some of them may be a bit too technical for you.