It may become very difficult to overcome this as quite often a persons self image has LITTLE connection to what they actually look like - anorexia sufferers are a prime example. You may want to see about a doctor, psychologists are VERY familiar with this issue.
I have an opposite view, despite being obese for over 25 of my 50 years I always viewed myself as being thin - I was even thin in my dreams. I was always SHOCKED when I caught a glimpse of myself in the mirror, it took me a moment to register who that fat guy was.
I don't know if EITHER mindset is better as I was always struggling with avoiding mirrors, pictures or even window reflections to keep from seeing myself and being disgusted by what I saw.
If this is really an issue you for you then PLEASE try and consult a mental health provider and see if you may be helped in getting a more accurate view of your self.