Don't flatter yourself !!!

JeanWhat is that noise ?
HarryWhat noise?
JeanI hear a girl saying "Harry, honey. Please pick up the phone, you stud..."
HarryOh! That's my new cell phone. It must be my latest squeeze, Jane.
JeanJane ?
HarryYeah. She's totally into me. She calls me all the time.
JeanDon't flatter yourself!
Explanation :
If someone says to you ' don't flatter yourself ', they mean that they disagree with your good opinion of yourself.
Example :
I flatter myself that this campaign will put an end to the war.