Feeling totally heartbroken...

So 5 years ago my boyfriend and I split up when I moved away due to distance. He ended up having a very causal relationship with this girl. He ended up leaving her because he was still hung up on me and then came back to me shortly after that. She ended up causing a massive drama filled mess within their group of friends (I heard about it from my own friends it was so bad and they hardly knew her). In the end she lost the friends due to being 'crazy, narcissistic and attention seeking' (their words- I've never met her). When my boyfriend and I got back together he told me about her and it hurt me a lot, but I let it go because we weren't together. Its been years since I've even thought about her. My boyfriend and I got engaged and are very happy.

Well, last night I got a picture over Facebook of my sleeping fiance her in bed together. Its captioned 'if you think your 'fiance' doesn't think about me your wrong'. Now, to my knowledge this girl lives across the country (my fiance moved to where I live) and the picture is obviously old. My fiance looks young in the picture and there's no scar on his forehead which he got shortly after we got back together in a car accident. So I know it was taken when they were seeing each other. But, I'm still a total wreck.

I feel totally betrayed and heartbroken. Its worse than when I was first told about her years ago. I was supposed to see him tonight, but I cancelled because I don't know how to let this go. I've tried telling myself that she wasn't what he wanted because he left her for me and she wasn't good enough to make him, forget me, etc. That all worked before years ago, but now it's so much more real and hurtful. I feel totally crazy for feeling like this. He loves me completely and I love him. I think she's just jealous (which is so weird considering the time that's passed) as this was sent days after we announced our engagement. Can anyone give me some advice on how to let this go? Its not like my fiance has ever done something like this nor would he ever. Yet I'm letting this girl get in my head!