Reduce what you are trying to build as much as possible. Then reduce how much code you use to build it. Build something you can use as soon as you can. If you're writing a ton of code before this thing is getting used then how do you know that you actually need all that code?

Your plan should include time spent on fixing bugs and addressing technical debt.

I started a new passion project last year. I don't know if it counts as a huge project, but I could tell going in that it would be the largest and most ambitious solo project I've ever taken on. Given that this is a project that nobody is paying me for, I have the luxury of not having to give any deadlines, so depending on your situation, your mileage may vary.

Firstly, whenever I encounter a bug or an idea that I don't want to interrupt my flow, I make a note of it. I'm currently using a Github project board for this. You don't have to use any complicated features to do this, it can be essentially used as a todo list so that you keep track of the things you want to implement. These things should be broken down into small achievable tasks.

I have embraced the idea that this project might take me literally years to finish, or it might never truly be finished. There are a great number of successful software projects that are never finished! Linux, for instance! I've given myself the odd fleeting thought to how much time it would take for a v1.0.0, but never tried to calculate it. I've accepted that it'll be done when it's done. Especially given that I don't know if I'll be too busy to work on it in a few weeks time.

This way, by not worrying about when it'll be done, I'm finding myself in that flow state of just working on the thing that I want to work on right now.