MyOB- Is it possible for you to let go of an idea? The future is something that cannot be planned. We all have ideas of what we would like our future to be. Here is the thing- sometimes it happens in that manner and sometimes it doesn't. The thing is she plans on being in your future. Why does having the picket fence mean keeping her within the confines of that fence? Isn't it better to think of it as when she has been on a journey, she runs to that picket fence and throws open the gate and has you to share her adventures, her time, her love. She loves you. No matter what she experiences on the other side of the fence her love for you will remain. You are very brave in your willingness to understand that she needs to roam a bit. She needs to visit with another love. You will always be a comfort she seeks, a love she needs. You are also wise in knowing that not trying to work with it (polyamory) is more of a detriment than trying to preserve an idea of the future. yes?
MyOB she is not hurting you. She is being who she is. You are looking at it as a deliberate or uncaring behavior. Look at it as another side of the woman you love. Accept it as you would any characteristic. Would you prefer she go back to her unnatural state of monogamy and you be perceived as the one who is causing hurt? Would you think that is a fair assessment that you are selfish and causing your love pain? No I am sure you would not. You seem to be a selfless natured guy. You seem to be a bit unhappy or feeling inadequate at the moment. Her love is not the cause of your insecurity, you need to find what is. She is who she is. Find peace in knowing that she WANTS you to be part of all that she is. Sincere best wishes for happiness.