Bite off more than you can chew / Jack of all trades, master of none
Literally 'He who grasps much, squeezes little'. Warns against taking on too many things at once.
You have too many projects at once. You're biting off more than you can chew.
—I signed up for five courses at once. —Jack of all trades, master of none.
She learned that you can't do everything at once, and started to focus.