One of the hardest things for some students to grasp is exactly how much they don't know about a language. Therefore many people unwittingly put themselves into an incorrect level of learning. Challenging yourself is always a good thing, however if you believe yourself to be more fluent in your language than you actually are, then you run the risk of burning yourself out with all the difficulty.
One of the quickest ways to help solve this problem is to take one of the many free online tests that help to place your level. BBC has a very usefull test as well. It is good idea to keep in mind that these sites are slightly biased, and may try and lead you to either use their products or buy their services.
You can also find simple tests like this in just about any language learning book that you could find at your local library.