We, as humans, are really strange. When we are faced with certain kind of danger or threat, our mind will sense it out and immediately send out some signals to block or to defend ourselves. Well, unless you are not human, ofcause (hahahaha). In that sense, you, my bro, would have to live with it.
First of all, the problem here is quite simple to solve. Two girls whom you have been going on for quite some time are basically madly in love with you. When you have chose one of them, it is natural that the other would be in the jealous overdrive. What you need to do now is to stick to one, ok?
Common, let's be frank. The other girl who is not happy with you is actually feeling quite bad with herself. She already have a boyfriend, how is she going to involve another guy? I think it is bad for her to take you for granted. I think if she wants to look for another guy, it will be just fair for her to break off with her boyfriend first, don't you think so?
Right now, I think you can't really think about it. Your mind is clouded with fear and anxiety. Fear not, bro as you are not the only one who is facing such a problem. I have successfully recover from that sort of situation before, ok? What you need to do is to stick to your guns, ok? Never let go!!! Be in peace!