I would agree with GalaGirl. It really does sound like you've put so much effort into the marriage for so long with nothing in return, that you are just completely burnt out on it. It seems that while I'm sure there are reasons you are with your husband, that he also very much is not suited to you in a few ways.
Ask yourself honestly if you are willing to live with him in a mono relationship for the rest of your lives and not have a lot of your needs fulfilled, and continue to do all the work when work needs to be done between you two. If the answer is no, I would recommend telling him that. Lay down exactly what your needs are, specifically how they are not being met, the time frame in which you would need them met to agree to stay (whether this be him changing to meet all of them or accepting you be poly if you will want that regardless, whichever you would need), and what you will do if nothing changes. I would also recommend you not go forward with anything new until this is settled. Managing more that one relationship is hard, especially when one is in a rough place, or if one is being done unethically. Ask your friend if he will wait until you sort out whats going on with your marriage either way, and then see where things go.