Lets be honest, we oftentimes expect great things from ourselves with little to no practice. We are lazy and childish in our artistic habits. We dream of creating amazing works of art, but when we first begin an art form we tend to be sort of...well, terrible at it. When my sister in law started dancing a few years ago, I'm certain she moved about like a clumsy ogre. Now she moves with the grace and fluidity of a hummingbird. The first time I made Alfredo sauce it was a wretched clumpy mess of cheese and liquid. Now, after lots of practice, I'm a great cook. When a friend of mine first started painting, her techniques left something to be desired. Now she has grown substantially in her non-career as a painter.
What I'm trying to say is, don't beat yourself up for being mediocre, and don't let it drag you down! Do lots of boring technical practice to hone your skills and you will eventually turn out exceptional work. Don't only paint things that you want to paint, paint things that you think will help you improve, like still life paintings (I am anticipating your angry sighs, still life is the bane of our existences). Or if you sing, practice your vaccai and scales every day. If you are a cook, repeat a recipe with your personal changes until the recipe is amazing. Second to practicing, you must have patience and be understanding enough that eventually (if you keep practicing), you will be great at whatever you do.
Don't let your dreams of greatness weigh down on your expectations, instead let them power you forward like a goal for the future. Good luck!