I have always struggled with writing music. My first year in Music Theory class I got the comment, "It sounds like a monkey playing a Jack-in-the-box." That definitely kept me away from composition for several years, but now I'm starting to experiment again.
My favorite quote from J.S. Bach is, "Anyone who is equally industrious as I will succeed just as well as I have." It made me realize that in order to write really good music, you need to learn from your experiences of writing mountains of mediocre and not-so-good music. Not every peice Bach wrote was a masterpiece, but that didn't keep him from writing new music every week!
So here's my advice to you. Pick up the pencil and manuscript paper and try to write a thirty second piece of music. Write for an instrument that you are familiar with, make two or three parts if you want to spice it up a little. Don't try to create the music on the paper, but instead try to write down the music that you are creating in your imagination or on a keyboard.
Write with a purpose. For instance, you could write down an articulation and scale exercise that you made up in the practice room, making it an etude. If you are feeling melancholy, write an expressive melody with simple harmonic or rhythmic figures underneath it. It doesn't have to be perfect. If it's not very good then don't show it to very many people, but use it for your own good by identifying the parts that you dislike.
Writing music is like making a sculpture, no matter how disfigured it is there is always some beauty to it!