How to show her that she's beautiful?

There's this girl that I've liked for a long time. She has a boyfriend, but he really isn't a good boyfriend, he's pretty much just an overall douche, and she doesn't stand up to him. This girl is amazing, she's of course gorgeous, intelligent, sweet, strong, generous, I love her with all my heart. But she's extremely insecure. I don't know exactly why, but obviously there are some underlying issues. The only thing I really know about her life is that her brother died young, and nobody knows how. She rarely talks about her father also. She always has to be liked by everyone, she never changes herself though, she'll just beat herself up over something that bothers her. I tell her she's beautiful a lot but she never believes me, I need to do something amazing to show her that she's actually amazing.
How to show her that she's beautiful?
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