I wrote this answer to a Yahoo question from a young person suffering with low self confidence, and it was chosen as the Best Answer - so I thought some of my readers here might benefit from it too.
The reason why most of these answers won't help you much, is that they involve what you'll consider to be lying. It's no good telling yourself you're not ugly, when you're pretty darn sure that you are, and the mirror seems to agree with you.
And it's no good hearing that no one cares, because YOU care, and you can't accept that THEY don't.
So what's to do then?
I encourage you to look very carefully at the people you consider to be not ugly. Look at their facial features - are they perfect? Think about your overall impression of them - do they come across as confident? Chances are that many of the people who you think look OK are in fact far from perfect, and that their good looks are actually down to careful grooming and a confident manner - standing straight, smiling, making eye contact etc.
When you look in the mirror, chances are you don't see those things becuase you don't do those things. So I would encourage you to start doing them today.
Confidence is a chicken-and-egg thing. You'll be confident when people seem to like you, but they won't start seeming to like you until you are confident.
To break this cycle demands a rational understanding of what's going on, a focussed attention to your thinking and to events in the world, and above all (the killer) taking risks - which needs courage - the last thing you feel like showing when you feel so down on yourself.
But it can be done.
There is a lot of free stuff on my website which can help you.