Phew this seems to finally be clearing for me. Thanks to everyone who has posted something. It does help me to have the biblical stuff interpreted in such sensible ways. Serialmonogamist your little post seemed to hit just the right note at just the right time. Then I went to an AA meeting last night - step 6 Being willing to have God remove all our character defects. It seems the more I go into the 12 step programme the more it opens up. I thought this step was primarily about prayer but it's about being aware of my defects and then changing my thinking.
I said my difficulties could have been about ego and I think they are. I think pride is my greatest character defect and my ego will find any way it can to feed it. Having a partner who also loves someone else attacks my pride at a very deep level.
I don't go to church very often because it can set me off into floods of tears. I've always found it really puzzling that there are all these happy, joyful people around me singing about the wonders of the Lord and I seem get such a different experience of it. A hard work experience. It makes me feel like I must be one of the worst of the worst. Maybe that's why AA works for me, no happy clappers just people who realise they have issues and are willing to look at the really hard things about themselves.