Welcome back...I would urge you to ask your husband to really consider being poly as a unit. I'd suggest you give him some time to get over the initial shock. It seems to me that your husband his trying to give you the option to choose him or hes walking..again this is his initial reaction which in most cases is not a rational one. I would tell him that his love fills you up and there is no one that will need to top it off and that its all him....but that having another person that can love her will bring in a different cup which does not interfere with his. Poly counseling is a great way for him to express his emotion and over come fears.
DO NOT BE AFRAID TO BE WHO YOU ARE...I wouldn't let his discomfort put you in a box that prevents you from exploring who you are.
You and your husband have YEARS of love and companionship I'm sure if his is willing that you and he can work it out...