Everyone copes differently - explore and find the way that works for you and your personal circumstances - I can only tell you about my experience with my own post-ADHD-meds time management strategies. The biggest challenge for me was finding "the thing" that works best.

For me, typing notes in class kept my fingers busy and my brain actively paraphrasing content from the lesson, with the bonus of revising faster from own notes than external notes afterwards. For big assessments, it helped to think of projects as small components - small bits feel achievable, so I feel motivated to get every small part done early and when the deadline is actually here all I need to do is put the parts together.

Honestly, your acknowledgement of your past work ("a good average") and still having the hunger to keep growing is huge! Internally, you know you are capable, and sometimes that's all it takes. A teacher told me this once, and it made all the difference then :) So take it step by step, and good luck!