Sometimes ending a relationship is the best and most respectful thing you can do for each other. It is hard but not every relationship is meant to last forever.
You sought out poly because there were problems you thought having extra sex partners would solve, rather than solving those problems first. When your wife said to you that she wasn't sure if she wanted to be married anymore, that is a statement that deserves respect. That, and the fact that sex is unsatisfactory for you both indicates a real need to heal and get healthy.
I am glad you decided to work on the issues before thinking about involving other people. Unfortunately, I don't see how much progress you can make without a third party guiding you, such as a counselor or therapist. If you really want to work on this together, that may be a point you should insist on. You want to know that you did everything you could, whether you end the marriage or continue.