Hey!
Try not to worry too much - three days for a single application isn't unusual at all! A good-quality application can take quite a bit of time, especially if you're tailoring your answers and revising them to ensure their clear and consistent.
It's actually a really great sign that you're putting in the effort - most strong applications aren't done in one sitting. Over time, you'll probably get faster as you reuse parts of past applications and refine your approach / writing technique.
If you're feeling exhausted (and it definitely is a tiring process!), try breaking the process into smaller stages (research, drafting, proofreading) and spread them over a few days. This helps make the application process feel more manageable.
You're definitely not doing it "wrong" - it just takes practice to find your own rhythm