This is especially applicable to teens. Your writing is constantly evolving and changing. Something you write now may seem underdeveloped three months from now. That for me is why nothing ever seems to get written - I take a look at what I wrote last month and realize that it could be so much better. So, naturally, I try to edit out the bad and add in the good, and after a few days I realize that I haven't actually added on to my story, I've just messed with what I've already done.
There is only one solution to this, and it will be something you will strive to do throughout your young adult life and into your old age: find your voice. You have to realize that what you're writing now is not going to have the same voice as the story you write a year from now, because maybe now you're reading The Fault in our Stars, so you sound like John Green, but next year you'll be reading a book by Tamora Pierce. Separate the voice you read in your favorite books with the voice that you naturally write with. Only then can you fully embrace your own style of writing, find your voice, and tell the world what you're thinking.
How to find your voice:
~ask yourself: does this writing have the same voice as what I wrote yesterday?
~reread something you wrote that you really like and speak the words aloud or in your head
~take a break from reading everything for an extended period of time (I recommend a month) and write every day you can so that your real voice will show itself