Firstly, I'm sorry you are going through this. It is such a horrible feeling to discover that your partner has been having an affair.
I found out about my H's EA with a work colleague 6.5 weeks ago now. Everybody is different I know, but I had to contact the OW and get her side of the "truth". That might not be the right thing for everyone, but there is no way I could have not contacted her, personally.
Yes, you absolutely stand to hear very unpleasant or hurtful truths - but those truths exist whether you hear them or not. Some people are happier not knowing absolutely everything. Not me, I needed/need the whole truth.
I would ask or tell your H to leave for now. This gives you some space to just think & make independent decisions without him being there sulking or trying to "explain". Then, when YOU are ready, sit down somewhere neutral with a list of questions you want answering and go through them one by one. If he is in anyway unwilling to do that, or you feel he is lying, then that would be the definite end of the relationship for me.
I know for a fact that my H's affair went no further than sexually explicit messages & topless photos of her - but I am still pretty sure our marriage will not recover, as I will never be able to trust him again.
In your situation - where he carried on with her even after D-Day, I'm pretty sure it would be over for me already.
Whatever you do, please be kind to yourself
. Do you have any RL support?