Beautifully written. It's hard to pinpoint when that shift happens but you need to decide if it's worth it to invest the energy to try to fix things to how they were before, or if it's better to move on and find another soulmate. It sounds like you maybe are leaning towards the latter and I can't blame you if she cheated.
Rather than let others decide your fate, be your own master. Cut it off if you see the end and it's grim. Your future, happy self will thank you for swift action.