EX'S. why does my boyfriend still talk to his?

So my boyfriend that I have been dating for 2 months still keeps in contact with his ex girlfriend. They have mutual friends so I realize that they will definitely cross paths with one another, but why does he invite her to his birthday and continue to text her? I do not understand for the life of me. They stopped dating half a year ago and came out of 3 year on and off relationship...on and off because he cheated on her or something a couple times they fought I don't really know the whole story. ONE story I know for sure is that he cheated on her with her best friend. Now my question is. Why doesn't she talk to her best friend any more but still talks to him!?!?!?!?!?

IT MAKES ME SO MAD AN CONFUSED. she must still love him or something, no? He has hurt her so badly but she continues to keep in contact, and he knows it bugs me so why does he text her especially when I'm sitting beside him. Does he just not care about me? or is nothing going on so he thinks its okay to talk to her.

Help me. What do I do? I know I don't have the right to tell him to stop talking to her but it makes me angry and hurt. I have told him how I feel and that it may be jealousy but every time I look over and he's talking to her I get in the worst mood and I'm tired of feeling this way.
EX'S. why does my boyfriend still talk to his?
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