How do you find the motivation to continue on large projects?

What helps me with such big projects is cutting it up into small pieces. When I want to build a city I like to build house by house. If I start by laying out a plan for all the building I know I will get overwhelmed and lose motivation. So I start by building 1 house, only when that's finished I will think about the next one. If I get any inspiration while building I write it down for when I lose inspiration.

Also: having multiple projects going on at once. Whenever I'm bored or lose motivation with one project I can work on something else and return at a later time. It can be something completely unrelated or not. If you are flattening a large piece of land you'll need a lot of dirt, so whenever you're bored of flattening you can do something else like gathering dirt. It's still working on the project, but doing a different part and it feels like something new. Or you can start building houses, gather materials for the houses, gather inspiration...

I like to watch a movie or listen to some music while working on long and grindy projects. I'll put on something I enjoy and play at the same time. That makes me less bored and keeps my motivation high. I'll try to put aside some new content for when I work on these types of projects so I'll want to work on them because that way I get to watch the video I've been wanting to watch all week (as an example).

Hope this helps!