Gnaw at
Definition 1 of 2
Pro Tip 1/3Emotional Impact
Describes a process that causes continuous distress or harm over time. 
The insult from earlier was still gnawing at him hours later.
Pro Tip 2/3Persistent Effect
'Gnaw at' indicates a gradual, often unnoticed problem that worsens. 
Rumors about the company's bankruptcy gnawed at the employees' morale.
Pro Tip 3/3Metaphoric Use
Metaphorically connects to the literal act of an animal chewing persistently. 
His failure in the exam gnawed at him, like a rodent on wood.