If you can write more than two sentences after a day of staring at the computer screen you're already leaps and bounds ahead of where I was at when I was 17. That said, there was a time when I, too, felt like my writing was getting worse instead of better. The problem came because I was researching how to write well, and all these "rules" popped up that changed my focus from the story (my imagination) to how the story was written (prose, grammar, etc.). My new stuff felt dry and boring whereas my older stuff felt free and fun when I reread them. If this is similar to what you're going through then you need to find a balance: write the first draft first and don't worry about any of the "rules." Always remember they're more guidelines than rules. For probably every writing "rule" that exists there's an author who successfully broke that "rule" and wrote a well-loved story. Find your story first, and then worry about how well it's written. When you get to the point that you need to write it well, one of the biggest things that helped me was lurking on this forum and reading EVERYTHING. Reading others' critiques and the pieces they were critiquing helped me tremendously--even more than doing critiquing myself. I also echo the ideas of others that writing shorter stories can help a lot. My first completed work was a short story and I learned A LOT from writing it and from the critiques I received for it. Good luck, and keep working at it! I couldn't write worth squat when I started, but through hard work and determination I've gotten to where I feel like I can actually write a decent story.