Welcome to the boards.
Have you sat down and discussed exactly what you each expect from your relationships?
It reads to me that you expected him to stay in contact with you and felt abandoned because he didn't. I'll also hallucinate that he expected to not have to stay in contact with you when off with her.
That's the sort of thing to negotiate beforehand. You have to describe what you need to feel secure so that everybody knows and everybody can negotiate how to specifically provide that. He also has to describe what he needs and everybody help find solutions. And she also has to...I think you get the picture.
The only way relationships work well--particularly those involving more than two people--is for everybody involved to offer up what they need so that everybody else can help provide it. It's no place for magical thinking of any sort ("if he loved me, he'd know/do X")--relationships require open, honest communication, at all times and under all circumstances.