It is still very new and raw for you. Be there for her but remember her journey and experience is NOT yours.
You both are in pain (rightfully so) try hard to see the instances as separate events because they are.
We are all flawed human beings and we all make mistakes some more harmful than others.
I am so sorry you both are going through this. Take one day at a time and remember to be kind to yourself. Allow yourself to feel all the emotions that come along with infidelity. Be mad/sad/pissed/scared/confused, etc. You have to feel them all to get through them.
Your question is a reasonable one but only the WS can answer the what, the why the how.
Stay strong. You are not alone.
Prayers for peace in your heart.