First I want to say, please take her reasons with a pinch of salt. Though she will have justifications, we must never forget people cheat because they think they are entitled to. Whilst some of her reason likely played into her reasoning 'And finally to improve OUR sex life and give her ideas or something like that' is just utter nonsense.
Don't leave. This would be a mistake. What I would advise is you have a calm conversation with her and explain that you need time to decide how you want to move forward. That you want to forgive her but you are not sure you can. That you think her leaving for a while might help. If she still refuses tell her - we need to stop talking. Live as room mates but leave each other alone until you've had time to think. To not come you unless it's regarding the child. Separate rooms etc.
I must admit, in response to 'I think this is what you wanted anyway and now you have an excuse to leave me' - I would respond: 'This is the last thing I wanted. I wanted to be with you for the rest of my life and now you've made that nearly impossible'. Firmly push the blame back where it belongs.
I'm sure she doesn't mean it but I think this is single handily one of the most painful, blame shifting response she could make. She's blew up your life, your view of her and to some degree your family - but it's your fault your considering leaving?
She needs a wake up call and fast.
Irrespective of the practicalities of divorce or whether you want to work through it generally, It's imperative you show her consequences to her actions. She is just hoping you rug sweep this all away, too many couple do this and it never works. You need to address the feelings or divorce. These are the two paths.
I personally think separation and seeing other people is the only way I could potentially reconcile with a partner. It's the only way you can have any semblance of rebalance the relationship. We all agree that their is no such thing as fair in life but it does rebalance the scales somewhat.
Further to this, it can clarify if continuing the relationship is truly what you want or if you are only desperately trying to make it work for fear of change. ~
What of her affair partner? Do you know of them? Are they still in the picture?
As a fellow UK-based member, have a pint and remember... this is all on her.
[This message edited by DRSOOLERS at 10:11 AM, Monday, January 20th]