Try to be patient with yourself.
You've had 20 years to get used to being a monogamous married couple with your husband. You've had 2 months to figure out how to be with your husband with another partner in the picture.
That's less than 1% of your relationship with your husband, to adjust to a completely different relationship paradigm from not only your marriage, but everything society and media has spent your entire life programming into you.
Even less considering a chunk of those 2 months you didn't even have a name for it.
I don't think this kind of flip flop thinking is uncommon. I know I went through a similar process in trying to reconcile the idea of my wife being with other men, and that process did literally take months for me.
Most people are their own worst critics...so be gentle with yourself and allow yourself the time to readjust, and assume that there will be mistakes and challenges along the way...both for you, and for your husband, and for his gf. Try to be forgiving of all three.