I've heard that you can improve your self esteem in three steps: awareness, choice, and change. Be aware of where/when/how you feel incapable, make a choice to think of ways you CAN do or be capable, and then slowly change your initial reaction from the feelings of inadequacy to the trained thoughts of ability. It takes time, but there are ways to improve your self esteem. There are also ways that your husband can help you by being extra aware of how you are feeling and trying to pay extra compliments to you when you look especially nice or do something really well and things like that.
Have you considered seeking counseling or anything about your anxiety around people you perceive as more attractive?
This is a HUGE adjustment, and remember that you do have the right to live how you wish so if you don't think you can handle having a poly partner you can say so.