"You're too good for me" is a statement that should always be believed.
When a man says it to a woman, it usually means he doesn't expect to be faithful and doesn't want to be held accountable for his irresponsible behavior.
When a woman says it to a man, it usually means she isn't willing to put as much into the relationship as he is, or that she isn't used to being with a man who treats her well.
Sometimes the statement means the person truly doesn't see themself as deserving love and commitment.
Regardless of which reason applies, you will not be happy with that person in the long run. Thank them for their honesty. And leave.