That sounds hard, especially if it’s something you really want to do. Assuming you can’t take an evening top up dose? I work with several kids who do, for similar reasons.
My son has adhd and I think I might too. A strategy that helps us is to break down the task or instruction in our head ( actually, often out loud), using words. And then say the words back when we do the task. My DS has golf lessons and his teacher asks him ‘tell me what I’ve just asked you to do’ which really helps. His teacher is excellent and instinctively knows what works for kids like DS.
I do dog agility ( my Labrador has adhd too obvs 
) and my teacher knows we can only use three commends, and need to act out ( often actually going over jumps myself!) what needs doing in little chunks. She breaks the course down and then builds it back up for me. Again, I link each step to a key word each time.
The other thing I would say is that feeling anxious causes a big problem ( I have zero executive skills when anxious!) so I do breathing exercises before we start. My son has learned ( took months!) to count to three before he hits a ball.
You really have my sympathy. At dog agility, because of these problems, I often feel like the stupidest person there ( despite having a doctorate in educational psychology). But my teacher is very patient and has listened when I’ve explained I find this stuff hard. And I now the other ladies there well enough that I can have a laugh when me and DDog end up off course!
Good luck, please don’t let this out you off enjoying your classes.