As a side note, i'm not saying you should switch to polyamory to save your failing monogamous relationship. I'm saying people should realize no one can make you 100% happy. If you want more happiness than what one person can give you, you should seek partners who can complement your life if that's right for you. Multiple partners can offer different views and strengths to your life. Maybe one partner is an awful mother, but another partner is a wonderful mother.Generally, "relationship broken, add people" is not a recipe for successful polyamory.
Someone may expand on this, but I'm just about to dash out the door to work.