<human>: I'm gonna keep this short. When we started dating, he had this best friend that is obviously a girl. I'm a little anxious about it but he reassures me that "she's like a sister to me" and explains to me that they are best friends and that she is bisexual. 

So I kind of calm down about it and try to be friends with her the best way I can. Well, now that we're not together, someone came up to me and asked me if me and my ex were still together because they saw a post with him and another girl with hearts all around the picture and they were concerned. 

Well, the girl in the picture was his "omg she's like a sister to me" 
best friend. Now wait a second... If this bitch tells me not to worry about somebody and then goes off and dates them, not to mention hang out with them the night we broke up.... 

What do I do here? What can I do to cope? This hurts so bad and I'm actually scared for the next time someone tells me not to worry about somebody, because of what happened this time. Advice is much needed.

Can you summarize the given text in a few sentences?
<bot>: Ex is dating the girl he told me not to worry about. How do I cope with this and not let it wither out my trust in others who tell me not to worry about certain people in their lives?