I feel like it's just always this way... with anyone that is successful. It's not a bug, it's a feature. I don't think you're broken like the way that you describe.

The key is to never let it go to "nothing". There's ALWAYS a negative consequence to shutting down. It doesn't come from outside you, it comes from within you.

If you let yourself have enough shitty days, then eventually you'll just become a shitty person. Don't take that first step. If you see goals that are unclear or timelines that are far into the future, then CLARIFY those goals. Pull those timelines back. So what if it's due in 4 months? Can you get it done by the end of the week? If you can, why wait? Just get it done!

Turn external deadlines into internal deadlines. Why should anything have to wait one second more than it has to? Complete assignments as you get them, not as their deadlines come up.

Turn external consequences into internal consequences. Look at your day. Evaluate how you did that day. What could be improved? Where are you struggling? Try to do better the next day.

Honestly, the anxiety has a purpose: getting you off your ass! If you had no anxiety, you would be like a capybara the whole day: just a big lazy baby. The point is that you're not just a chubby, lazy baby! You're a responsible adult, dammit. Even though there is always the temptation to just be a gentle soft chubby boy and sleep the day away, don't give in!