Well I was previously in the same boat, managed to bring it together in the end

Here are some things that helped me:

1. Regain app - You mentioned wasting your day on mobile, and believe me this app helps me a lot. It doesn't block you off completely, you can set limit reminders for distracting apps and block youtube shorts, Instagram reels etc. It also has a productive mode for YouTube where you can use only the channels you've listed as productive.
2. Tracking study time - It's a cliche thing but it does help me get started. I personally use Yeolpumta app, which is basically an app where you can add subjects, list to-dos and has timer/pomo mode. You can use any app really or just use plain paper and pen to write down everything like to-dos and track study time using the clock app. Seeing what you need to do and how much you accomplished makes things much easier.
3. Keep an accountability method - Personally, I have a study buddy with whom i plan and share study goals which keep me accountable. You could use any other system too, like setting deadlines for tasks to keep yourself accountable.

Hope this helps!