so my SO knows that I don't like when he texts or Snapchat this girl (his "bff") while he's at work…

so my SO knows that I don't like when he texts or Snapchat this girl (his "bff") while he's at work she only talks to him when she's single and we've gotten in arguements about this but he started doing it again and I'm so frustrated and feel 2nd best yet again I already feel unattractive and gross this makes me so upset what do I do? 😢😢😭😭