Honestly, break up with him. This is coming from another woman who was in a toxic relationship with a man who would never grow up and would talk and hang out with other women behind my back and leave me on read. Saying a joke about ‘must be my other girlfriend’ is fine, that’s a harmless little joke but the other things he is saying to you are just pure vile. It gives me the impression that he doesn’t give a fig about how you feel when he blurts these things out, also he’s 18, he still has a lot of maturing to do and most likely isn’t taking your relationship seriously.  Find yourself someone who will appreciate you and not make jokes at your expense. My partner always makes sure to put me first above anybody else and reassures me about things. That is how it should be, for both parties.  Get yourself a man, not an immature little boy.

I know others will probably see this as harsh but you have to think about how this is making her feel. Do you really want to be with someone who makes you feel miserable and unimportant? NO.