Spread yourself too thin
Definition 1 of 1
Pro Tip 1/3Time Management
Refers to poor time management by trying to do too many things, which affects performance. 
I was spreading myself too thin managing three projects, and my work quality declined.
Pro Tip 2/3Negative Implication
The phrase implies a negative outcome and emphasizes the inability to focus properly. 
Since you've started volunteering on weekends, you're spreading yourself too thin and making more errors at work.
Pro Tip 3/3Personal Limit
It highlights a person's limit and suggests they're exceeding their capacity. 
You're planning to learn four languages this year? Careful not to spread yourself too thin.