Yes, I do believe her feelings of insecurity stem from low self esteem. I won't get into it on a public forum but suffice it to say she has a history of abuse, although its been many years since shes been in that situation.
I actually dont think she is more interested in my husband. Before we got together her relationships were primarily with women, so she has very little experience with men. I think the envy part is her seeing how easy it is between he and I. We have been together for a decade and have reached the point where we hardly ever fight because we know each other so well. We are deeply in love, and it shows.
The thing is, we are each also deeply in love with her. She brings so much to my life and I love how giving she is. We have been together for a year, and plan to stay together. We are constantly talking, and she understands that her jealousy feelings are hurting our relationship. She is in counseling and I've tried to find a counselor for all of use, just to talk it over with a professional, but poly-counselors seem few and far between.