I've been worrying about this for quite some time, as I have a really short attention span aside from only the best times, and I sturggle to stay focused on any one activity for any sustained length of time. I would much rather go out to the doctors or a psychologist or something, but I really don'tthink my parents will believe me, and they'll just dismiss it (I'm quite often a pretty paranoid person).
It's also been getting in the way of my school a bit, as I'm starting to notice that I can't focus on subjects that don't immediately interest me. I'm doing well, mostly, but I did really bad on an english test due to lack of time management, and I had heard that that may be a symptom. I also get really easily distracted, even by my own thoughts.
I'm really worried, because I don't want it to worsen.
Please, anyone with or without medical knowledge, advice, please.
Could be, could be a lot of other things. It could be that as your life is getting more complex you're not as able to cope with your pre-existing ADHD. It could be something else. What you describe does sound like ADHD, but it also sounds like being a teenager - it depends on how much it's impacting your life and how much you have to go above and beyond what your peers are doing to manage the same level of functioning.
Good news: The same things that help with ADHD will also help with most everything that causes disorganization and distraction! Aside from really serious disorders like bipolar (don't stress about being bipolar - it'll get pretty obvious). So there's no need to rush to a diagnosis.
There's lots of good free advice about how to be organized and manage executive function problems (i.e. time management, loosing track of things, short attention span). I recommend Lifehacker as a good blog. 7 Habits of Highly Effective People/Teens is a good book to start with.
Basics that helped me:
1) Practice mindfulness. Take a few minutes to be still and just feel where you are - the textures of your clothes, what the room you're in is like, etc. This helps ground you in where and when you are instead of getting lost in stuff you're thinking about. It sounds like nonsense, I know, but it helps A LOT. Do this at least once a day, and whenever you notice yourself getting way too distracted.
2) Take a short 10-20 minute walk every day without headphones or anything. Having someone along to talk to you is also good. Another grounding technique.
3) Go to bed and get up at the same time every day. This is to teach your body a regular routine so that your hormones and energy stabilize.
ALSO 3a) No electronics, books, watching TV or other entertainment in bed. This is to teach your body that bed is for sleeping, so you don't lie awake in bed, which is super frustrating. If you read / whatever in bed the adjustment can be hard but it's totally worth it. I went from not falling asleep for an hour to falling asleep in 15 minutes.
4) WRITE ALL THE THINGS DOWN. I can't overstate how helpful it was to just write everything down. Abbreviations make note-taking go faster. Carry a small notebook and pencil so you can note down things people ask you to do if you have trouble remembering. Some people will laugh at you, but some people will laugh at you for anything because they're just jerks, so don't worry about it.
5) Ask teachers for help. They're often willing to remind you of which pages the assignment was on, when the paper is due, etc. They generally want students to do well.
To get a real diagnosis of ADHD there's a long questionnaire to fill out, one version for you and one for your parents. I'll see if I can dig up a sheet.
http://www.aacap.org/AACAP/Families_and_Youth/Resource_Centers/ADHD_Resource_Center/ADHD_A_Guide_for_Families/Home.aspx and
http://www.aacap.org/AACAP/Families_and_Youth/Resource_Centers/ADHD_Resource_Center/Home.aspx are good places to start researching for a diagnosis. Remember - a lot of the symptoms of ADHD happen sometimes to everyone and that's normal. When they happen so much they cause repeated problems, they need further investigating.
Even if you have ADHD and not something else, you'll still have to practice mindfulness, work on organization, etc. MEDICATION HELPS BUT DOESN'T SOLVE IT - ADHD is a different way of processing and remembering, not something that a pill can fix. Don't rush into a diagnosis thinking that you can take a magic pill and be perfect. There are a lot of resources you can tap before heading to a doctor's office, and any decent doctor will tell you to make lifestyle changes right along with the pills, so you may as well start with improving your habits first.