Like a dog with a bone
Definition 1 of 1
Pro Tip 1/3Persistence
Used to describe someone who refuses to stop, even when facing difficulties. 
She pursued her dream like a dog with a bone, never giving up.
Pro Tip 2/3Singular Focus
Implies focusing on one particular issue or goal to the exclusion of others. 
The detective was like a dog with a bone, focusing only on finding the missing painting.
Pro Tip 3/3Potential Negativity
Can be used negatively to describe someone overly fixated or obsessive. 
He's like a dog with a bone with these conspiracy theories, and it's getting out of hand.