I tell myself that my stretch marks are the equivalent of a free commemorative tattoo for the time I was carrying my son. Dumb, but it kind of helps.
I like the pin going around Pinterest saying something like, "Your body isn't ruined; You're a tigress who's earned her stripes."
Having a baby is one of the hardest things you can do to your body. Not just delivery, but carrying as well. Try to focus on that fact. I have such bad stretch marks that I literally have purple stomach/hips, so I have to remind myself that my body has been put through tremendous stress and that it'll take time to recover, just like any other part of life. :)