Why is getting started the hardest part even when you actually want to do the thing

Today I had the time, the energy, and zero excuses. Everything was set up to actually get something done.
Still spent 40 minutes on my phone before I typed a single word. Not because I didn't want to work... I genuinely did. It just didn't happen.
I've realized the longer I sit with a task without touching it, the bigger it gets in my head. At some point it stops being a task and starts feeling like a decision I have to make over and over again.
I've been trying to fix this by actually organizing what I need to do before the day starts... not just a random list but actually knowing what's priority one the moment I sit down. Still figuring out what works though.
Curious what others are using to manage this... not just to track tasks but to actually know what to focus on first. Because half my problem isn't the distraction, it's sitting down and not knowing where to even begin.