Ex has new girlfriend

I really am over him as much as I can be for someone who tends to hang on to the past, but because this is my first breakup, it's really hard for me to see him with her. I've never experienced this before and I get this awful anxious, nauseated feeling when I think about it. I feel like she's so much better than me and I'm not as worthwhile as she is. I want more than anything to put him in the past and not think about this anymore. And advice or words of encouragement would be very appreciated because I feel so alone.