Ex-boyfriend Problems

My boyfriend and I have been together for two years. Before we started dating, I dated my sister's best friend (he's a guy). My sister and this guy are still best friends, and he often comes over to our house, sometimes while my boyfriend is there. My boyfriend doesn't seem to understand that to me, my ex is nothing more than a friend of my sister's, and I don't care about him anymore, but any time my ex comes over, my boyfriend gets all mad and either leaves, or is in a bad mood for the rest of the day.

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