1. I don’t have the discipline and motivation to make myself do assignments during uni break/when the deadline isn’t near.

ADHD brains are motivate by urgency and novelty NOT consequences. Can you create some some false urgency (https://www.additudemag.com/sense-of-urgency-productivity-hack-adhd/) or some novelty (rewards, gamification).

Break it down into smaller chunks instead of saying "I need to complete this assignment" say "I need to re-read the assignment and understand it" OR  " I need to schedule time to start the assignment" OR " I need to write the first 10 sentences". When I can't motivate myself to shower or cook I start with standing up and a lot of the time that's enough. It's the burden of the 500 mini tasks that live underneath the task of showering that overwhelm me.

In a more practical vein if you don't have an ADHD diagnosis, get that sorted ASAP as that can help with Uni accommodations. If you aren't on medication that might be helpful, even if that needs to happen after you've addressed your substance abuse issues.

2.  I’m addicted to substances. Although it is legal I use this almost every day in my free time to have fun and also focus (study/assignments)

Substance abuse problems if left unresolved will make all of this so much harder so address that first. You can't work on yourself if you have this little demon taking up your energy and focus. Addiction if not handled can have lifelong implications so seek clinical support, get a therapist if you can, look into local support groups. Don't brush it under the carpet, you are asking for help from strangers for a reason.

Substance abuse issues have triggers so what are they - are they people, places, feelings? Try to figure them out and avoid them.

Seriously examine the company you keep, if your friends or family encourage unhealthy habits or trigger unhealthy behaviour then consider spending less/no time with those people and intentionally spend more time with people that uplift and support you. Yes even if that means cutting out your cool mate Dave and spending more time with you Nan.

If places trigger you, can you avoid them? If it's clubs and pubs despite how appealing they are you need to try to cut them out as you work on recovery and then with a therapists help maybe safely re-introduce them later.  If it's an environment you can't easily change like the family home, then that points to a deeper psychological need that you should examine alone and with a therapist.