To me, achievement means committing to a task or job to gain something or complete a goal like going through school and education to get a degree which is the end goal. My biggest achievement so far is succeeding and completing military training during primary school. I also have stayed in Set 1 for my science lessons the whole of Year 7.
Achievement will be different for everyone, no one has the same goals or achievements. Focus on your own achievements now and celebrate even the small wins when you can because someone would probably love to achieve some of the things you have.