How Can I Improve Concentration Without ADHD Diagnosis, Considering Worsening Focus?

I am 22 and I m from the UK I have struggled in the past with my concentration but recently it has gotten a lot worse as I can t stay focused on a task for longer than 10-20 mins without needing to take a break. I plan on taking Ritalin or anything else that could help me concentrate for longer. I haven t been diagnosed with ADHD or ADD. And I don t know why my concentration has completely gone in the past couple of months and I don t know how to fix it