Aunt Maggie - the beauty of Mumsnet is that you're going to get lots of different viewpoints, some of which your friends won't give you, because they have an investment in you still liking them if they tell you what you don't want to hear. The converse is that none of us on Mumsnet know you, your partner or how you come across in RL. My best advice is - if you have a friend who will really tell you how it is - and knows you both, ask that person for their take on this.
Like you and Happy Woman, I think we all do things in life that we never imagined we would - for good and bad. We change so much over the years and none of us knows what we would do in a situation, however well we think we know ourselves. We certainly cannot know what other people will do, however well we know them. My H would never have dreamed before, that he could be unfaithful and neither would I. In fact, I thought I was more capable of this than him. This latter bit of self-knowledge did at least give me some empathy with his situation.
I never thought I could start on a path to forgiveness after this, but I'm profoundly glad I surprised myself and found reserves of strength I never knew I had. I'm proud that I turned out to be a more forgiving person and I feel rewarded every day for that strength.
Aunt Maggie, there will be people on here who will say that you should walk away and that no infidelity should ever be forgiven. Others like Mal will say that it could never happen to them. I would have been in both those camps once upon a time and all I can say is that I'm glad I'm in neither now. I have far more empathy now for the human condition and a much clearer view of my true feelings towards my H.
I also have so much more knowledge now about how a couple can protect their marriage from infidelity. Prior to my H's affair, we were so bloody smug it would never happen, it didn't occur to either of us to build fortresses so that no-one could get in.
The best work a couple can do after an affair is to recognise why it happened and to make changes so that the future relationship is "affair-proofed". This also meant some real honesty on my part - I now believe that both of us, at that particular time, were vulnerable to an affair opportunity. We're both pretty certain we wouldn't have gone looking for it, but if someone had wooed me with the intensity that OW did, I have to accept that I might not have resisted either. At other times, as my H knows, I did turn down numerous opportunities, but these were at a time when I felt great about myself and didn't need an outside boost. In early 2008 however, it would perhaps have been a different story....
This experience has taught me that the walls around a marriage need constant reinforcement. My H and I know now how to ensure that nobody can ever get throught those walls again. We're grateful for that knowledge, however painfully it was gained.
I think you're approaching this in exactly the right way - searching for answers and taking your time with such a monumental decision. The reward for you, whatever happens, is that you will grow as a person and this experience will teach you something. I feel sure that what ever you decide Maggie, it will be an informed choice and that you will choose what is the happiest option for you in the long-term.