I was diagnosed with ADHD at 24, about half way through my second degree. Prior to that I got suspended from my first university twice, then kicked out.
Definitely recommend finding a good therapist and notifying your university. Once I did they were amazing about organising extensions and helping me get organised. Having said that, the two most effective things I've tried have been meditation and a technique called noting. Will link you a couple resources I found helpful. I graduated with solid marks and wound up working in a university research department. All the best with everything mate, you'd be shocked at how much better things get with a few tweaks.
The Conversation - heaps of articles by Australian scholars on ADHD
Headspace guide to noting - plenty of other free guides on there too