Apparently from what you wrote Tisky, she is not happy in her couple, but how unhappy is she really and how is she able to actually cope with it ? You have to consider an important thing : you are free looking for someone, and she is engaged. That puts you in the waiting position while the choice is on her side. If she was sure of her love for you, she would take a clear and sharp decision, act, break up officially with her man and get engaged with you, but essentially that should be coming from her. However in the meantime, you are waiting. And you cannot wait forever ..
You know, there's people, more often out of unconscious suffering than calculated maliciousness, who vampirize others to fill their neurotic needs and make them carry the burden of what is going wrong in their life at their place. Is it the case ? I don't know. Is her love for you sincere but she is not able to assume by fear of breaking apart her family ? Did she used you to prove to herself she can still make a man fall in love and refill a worn down narcissism ? Is it a revenge to wake up the interest of her husband ? ...
But you have to get a grip on how ridiculously paradoxical it is to accuse of cheating an unofficial lover that you keep hanged to your decision while you are having it both ways .. Sounds like she may just have dropped all of her guilt on your shoulders for you to assume at her place and took a strategic retreat. From what you said, I think that guilt you can clearly shake it off right away.
But now you have taken some distance, and hopefully healed a bit, don't hurry, be easy on yourself and take your time to grieve the end of that relationship by following the wise advices all the good people of mapcore gave you above, have good moments with your children, concentrate on doing stuff, see people you like to spend good time with. And then, when you have recovered enough strength, I think it is important you ask yourself what was driving you towards a situation that was bad for you. It's always a good opportunity to learn something about yourself. And.. avoid it to happen again in the future. ( If you have the time and money, who knows, maybe a psychoanalysis would help ?)