I must second Jack’s response. Your boyfriend may either not be ready/mature for a relationship yet or just using both you and the other girl. It is actually a pretty good deal for him – he gets the attention and admiration of two women. Your anger is misdirected towards this other girl, when it is really the boyfriend who is stringing you both along and getting what he wants. He has both you and the other girl angry at each other while he gets what he wants from both of you.

Like Jack said, what it comes down to is loving yourself and setting boundaries. It is easier said than done, but I think you must give this man an ultimatum: me or her. Prepare yourself for an emotional loss if he chooses her, care for yourself, then move on to someone who only wants you.