From my answer to What does it feel like to have Body Dysmorphic Disorder (BDD)? on Quora
When a normal person looks at their reflection, they see themselves.
When someone with Body Dysmorphic Disorder (BDD) looks at their reflection, they cannot see themselves because they’re so preoccupied obsessing over their perceived flaws.I have a unique type of BDD where I see myself as much smaller and skinnier than I actually am.  (Sometimes called Muscle dysmorphia or bigorexia, this is opposite of the more common form anorexia where a skinny person sees themselves as much larger or fatter).  Regardless, the symptoms and phobias are very similar:
I spend mornings avoiding my reflection because I wake up feeling “smaller;” the sight of my deflated figure can ruin the rest of my day, sending me into a depressive spiral obsessing over my many defects.
I am extremely insecure about my physical appearance and am always seeking outside input to reassure myself that I’m not as “small” as I think I am.  The extremes at which I seek validation have labeled me as a narcissistic exhibitionist by folks who don’t know about my diagnosis.
I have a lot of trouble accepting compliments because I feel like people are lying to me.  I’m still grappling with the concept that I’m attractive to some people because I don’t feel attractive at all.
I belong to three different gyms and spend nearly half my paycheck on supplements, food, and anything that might help me escape being so skinny. 
I avoid going out and socializing because I might stay up too late and if I don’t get enough sleep I won’t be able to recover from workouts.
I’ve made considerable progress in the past couple years through therapy and reinforcing positive self-image.  I’ve had the most success with a daily self-portrait project that helps me appreciate the parts of my body I’m happy with.Nevertheless, my coping mechanisms aren’t a cure, and occasionally I’ll have a BDD attack and breakdown in depression.