I recently (a few days ago) cut my hair, from medium to short hair. I did originally have really long hair and just wanted a complete change.
A lot of people wouldn't take this step because they're worried about how it'll look. This is just to prove that having shorter hair, is not the end of the world.
The things you have to keep in mind are 1. It's hair, and it'll grow back. It's not just going to stay that length, it will grow. I would say, if you are unsure then cut it after summer and then by the time you get to the next summer holiday, you'll have your longer hair back.
2. After having it cut, try different styles. Make it wavy, curly, straighten it, or even put it up in some way. Don't confine yourself because you think that by having shorter hair, you can't do anything with it.
3. Make sure that when getting it cut, you also have some sort of texture/movement cut into it. I personally think that having a few layers put in, helps the short hair have some bounce, and just looks like there is more going on.
4. Finally, never put yourself down because of your hair. It's one thing out of you as a human being. No one is going to judge you because you think your hair looks awful. Life is not over after one hair cut.
I suppose you could just wear a hat at all times if you really don't like it- I'm kidding of course, unless you think that you could get away with it. But in all seriousness, the world isn't going to end, and if it's too short, it'll grow- just remember that.