It could be that your symptoms were not causing significant problems when you were 10, but are now as the demands on you get tougher. This is fairly common. Remember, diagnoses are given in part based on how impairing they are.
Also, psychiatric conditions are not like a missing limb, where anyone can see it. Doctors often miss them unless they specialize in that disorder. Medicine is like any other field, doctors have specialties. Even psychiatrists have specialties! If the doctor you saw was not a psychiatrist trained in diagnosing learning disabilities, you could easily have been misdiagnosed.
I would get a diagnosis from someone recommended for diagnosing ADHD and learning disabilities. Check out CHADD
http://www.chadd.org/, they are huge in the US and may also have resources for other countries. At the very least, they have good advice on managing ADHD and can give you a support network.
http://www.dodsonadhdcenter.com/ is a famous center for ADHD and may have advice. I don't know any European doctors, sorry.
Until you get a diagnosis and medication, focus on improvements you can make to keep your life on track academically. You sound like you're still in school, so academics are what you should focus on while you have parents to manage other things. The most important thing is note what helps you and what keeps you from being productive. This is different for everyone, so you need to find what works for you.
Here is what worked in my family. It will not solve everything at once, but it helped me (autistic spectrum + ADHD) and my sibling (ADHD) as we stuck with it, so hopefully some of it is useful for you:
Getting enough exercise and sleep is essential. Sometimes it's not possible, but try hard.
Get help making a clean and neat space to study. Do your study there, and don't do other things there. This is to signal your brain to focus on school work. Bedrooms are usually a bad place, because they have all your stuff and also your bed. Very distracting. Try to find a nice balance between not too isolated but also not right underfoot. Home offices are good, so long as the people in there will not always be bothering you and you have room to move.
Set aside time to study, enough time to get your homework done. The time limit helps you feel motivated, and gives you an end point so that you don't have a whole night of homework stretching out in front of you like a demoralizing wasteland. This time, and only this time, are for study and school work. If you find you aren't getting everything done despite trying really hard and focusing for that time period, make it a bit longer until you can get everything done. Don't go over 5 hours in one day, that's just too much. Focus on being more efficient if things take that long.
If you get distracted repeatedly in a short time, get up and run around or do jumping jacks for 5 minutes - DON'T go play a game or some other entertainment. This is to get your body stimulated and feel more energetic. Then study again. Repeat as necessary. Don't stop trying to study until your time is up. After that, don't worry about it. Keep doing this, and you will learn to focus on schoolwork in your study area, and to give your brain a rest when you are not studying. Stressing constantly about schoolwork is very counterproductive. In general, make sure you are getting a good amount of exercise every day so you can focus better.
Also, sometimes it helps to do things without sitting at a desk. Try solving problems while standing up. I read most of my literature books while lying on the couch. The point is to learn, not to look like a picture of someone learning.
Sometimes my parents read books with me and we talked about them at dinner to help me understand. Sometimes I just gave up and got the Cliff Notes. Ask friends for their notes. Ask your parents for help. Find tutors. Even people without learning disabilities do these things to help when they are struggling. You don't have to, and should not be, doing this all on your own.
Read advice on study habits, and be prepared to do things a bit out of the ordinary. For example, I learned algebra using number blocks and chess pawns (to represent variables) on a drawing of a balancing scale. This worked really well, where writing out equations did not. Once I knew it I could do the written equations, but I had such a hard time conceptualizing it until I got the scale set. I learned to draw models to solve geometry problems. You can learn! But maybe not in the same way as the rest of your class.
Read the textbook, even if you don't understand it, and try to summarize what it says. Repetition is key. The more times you hear or read it, the quicker it sinks in.
Take good notes in class. Needing to write everything down means no time to get distracted, and keeping up with the teacher means you have to think about what you hear and figure out the keywords. This helps you remember better than just hearing it. Also, tell your teachers you are struggling and ask if they could give you lists of key concepts so you can see if your notes have the important points. DON'T use this as a reason to not take notes - this is to make sure your notes are getting the important bits.
DEFINITELY make sure you are progressing on getting a diagnosis from someone trained in diagnosing learning disabilities. The stuff described above is a stopgap measure to help you learn. If you have ADHD, you almost certainly need medication. Trying to manage ADHD without medication requires a lot of support and accepting that you will just not be able to do some things. I have tried it both ways, and medication is nearly essential. ADHD is not such a problem in kids because kids have fewer responsibilities, but untreated ADHD in adults is the cause of a lot of addiction and other issues. You and your parents need to figure out what is going on.
All that said, learning disabilities are disabilities for a reason. Some things are just going to be very hard for you. It's crappy, but it's the way life goes. Don't beat yourself up for not being perfect. You don't need to get all top grades and do your chores perfectly. Do what you can, focus on your strengths and learn to make your life work for you.
