thanks again for taking time to add your comments.
We both agree that counselling is the answer. To be honest, I found it therapeutic just churning out my original post earlier...
One point I find possibly pertinent, is that when I asked her if the poly side of her has been present throughout our relationship, she told me that when things were going really well with us, she never gave it a second thought. So maybe it's a side of her that came out when things were going badly, when she felt taken for granted, and when I neglected her sexually (which I admit I did at times, particularly when my stress levels were at their worst).
Is polyamory something that can come and go like this - depending on the happiness of the relationship? From reading this site my conclusion has been otherwise - that it is part of who we are. Maybe she was just able to put it to one side when we our relationship was going really well. I don't know.
Once again, thanks