Seeking advice on how to answer a tough question

I hope this is the right subreddit for my query. I recently resigned from my dream job after six months due to an overwhelming workload, often working 10-11 hours a day and even on weekends. The company I was with is currently contracted for a project that will wrap up in December, and I left before its completion. Now I’m applying for similar roles with permanent contracts and anticipate being asked why I left before the project ended. Any suggestions on how to respond to this question? Thank you!