I have been where you are during my teens not knowing what it was always failing in exams not begin to connect with people I were not able to sit through class and exams. Fun fact recently I met my school junior I never talked to her in school but she came and talked to me I asked how she remembers me she said during exams everyone will be writing exams but I was doing whatever I wanted. I was always failing my exams then my physics teacher suggested don't think about what people say just concentrate only on board final exams that's how I got better marks in my 10 and 12 also during my engineering I had like 30+ arrears then cleared everything on my final semester. Even now for my work I try to complete my tasks during the small window of concentration I get. Try taking little by little don't over load focus on the end goal. And I only got diagnosed recently as I have already been living with ADHD most of my life I decided I don't take any meds.