The moderators will give you a good breakdown on steps you should be taking. Many of them are contrary to what you feel like doing. It's not about you, she is struggling with herself and she likely sees you as an obstacle at this moment. I can see this is very difficult for you and I hope you get through this, try not to get overly emotional in any way, be the best Father you can be and give her her space. I would also not rule out an affair that dismissively, when they turn upset at you for little things and start the proverbial falling out of love phase, it's usually because someone else has caught her attention and they might think they have an opportunity at a new life or a fresh start somewhere else. I am on the same boat as you and I realize in hindsight a complete personality change does not happen without somebody sort of catalyst.
As other posters will tell you, focus on yourself and making the best version of yourself, that can never be overstated
