Here's a few things I do:
1. I have two lists. One which has things I do regardless of how I feel. Another which has things I only do when feeling my best.
2. I time-box everything and stop working either when it feels right or the time expires.
3. I focus on one big project at a time between work and personal life. No more, no less.
4. I strategically underachieve in the area that doesn't need my direct attention or focus.
Life is very seasonal, sometimes work will be the most important thing to focus on for promotion season. Sometimes school will be the most important thing during finals. Sometimes side businesses can take off and need attention at any given moment. There's no right answers here, just continue to work on your unique balance.
You can think of it like a juggler with too many balls in the air or a kitchen stove with too many burners on. There's also ancient quotes about putting more wood behind fewer arrows.
Whichever way you like to think about it, less is more.
You cannot think of all your responsibilities demanding your attention in a parallel manner. You have to make a list of things that needs doing, or you would like to do.
Then tackle them one by one. When you are doing something, focus on that and that only. Learn to prioritize.
Then go down on your list depending on the priority, but "business" is tricky. Does it require you to handle unplanned stuff and needs dealing with people? Then it is going to be tricky.
Also, be aware that, the solution is different for different people. And you should reach your solution through a focused, thoughtful trial and error process.
Extensively using some note-taking method helps. Using a tool like ClickUp or Trello also helps. You can also check out the Getting Things Done method deviced by David Allen.
There is no silver bullet for your situation. You should try different approaches until you start to figure out what works. And it will take time. From a few days to a few months. And finding a solution to this should be your primary priority.
I personally think, that university should be the most important part to put time in here, because it should be the most fun in the present and most rewarding in the long term.
But the general rules and practises of staying focused still apply to your situation. Take a look at "Deep Work" principles and maybe as a fast and encouraging start, you can take a look at the analog principle of having a non digital notebook, a pencil and lists for "TodoToday" "TodoLater" and "TodoSometime" :-)
I was the same way when I was in my 20’s, I had work, university, and a side business and did all three mediocrely.
Now I plan one big focus per quarter. This quarter was “learn Chinese”, so I cut my freelance hours, signed up for 4 hours/day private lessons, and stopped drinking alcohol outside of 1-2 drinks on a Saturday because I realized even one drink at night caused me to be tired during the day.
The other key is I write _everything_ in my calendar. Even “Lunch”, “Gym”, “Laundry”, and “Call parents”. This forces me to not over commit any time.
Best of luck! I believe in you :)
Start from your goals and values. What do you want to achieve? Why?
You can't beat physics, so if you're running up against time constraints, you need to prioritize more. Let your goals and values guide you.
Side business at the same time, for the same duration, every workday.
Work every workday (lol).
Other hobbies I schedule similarly - make it so I don't have to think about when to start.