I don't quite know if you're too familiar with it, but I personally like to utilize aspects of the AGILE and Scrum pieces of Project Management whenever I could when working on larger and bigger projects.
In a nutshell, you need a balance between proper pressure and a goal to push. Usually, moving your goalposts is seen as a bad thing when it comes to your own projects (and it should be a red flag in general), but you need to also tackle your big tasks in small steps .. so those should be your goalposts.
A more practical example from me would be setting a personal deadline that is both achievable and realistic for you (and your team). Give yourself something like two weeks to do something that should seem basic, like setting up the foundations (movement in gameplay, your core gameplay flow, finishing an asset or a group of them, studying something new that'll come in handy, and so on). When that period of time is done, try to reflect on what worked, what didn't work, and try to plot your next few weeks so you can get something else done. The goal is to check back on your goals, reflect on your progress, and course-correct to keep setting up tasks you can reasonably accomplish so each stepping stone leads you closer to where you want to be.
These repeated deadlines and that pressure of "I need to get this done by then" especially on your terms and in your control means that you can find new challenges to keep that interest going (if the project falls through - you still have that knowledge and something to gain from a failed project) and to push forward. :)
If this is a bit much or too hard to try, I definitely recommend participating in a few Game Jams. These tend to be incredible periods of crunch for me that are super stressful - but fully capitalize on why it's so important to be able to adapt and reflect so quickly.