Hi, I am sorry that you are going through this. What you are feeling is absolutely normal. Just let yourself feel what you need to feel (anger, hurt, sadness, etc.), and then start looking, clinically, at what your options are. You are one of the 'lucky' ones, in that your H wants your forgiveness, and wants to work on the M.
Your H has broken your trust in him, and he is going to have to work on regaining it back. It's his responsibility, not yours. Your responsibility, is to decide whether you want to work on the M or not.
When I discovered my H's EA, I knew I had the option to stay or leave. So, I explored leaving - I went to a lawyer to find out what my rights were, the cost involved, etc., I went to see a C just to talk through the scenario of confronting him, and the possibility of leaving (and all the stuff that entails, like the children, financial stuff, where to live, etc.). Then, I looked at the option of staying, and the steps I would need to take in order to get to the point of being contently M. It's been quite a ride, but I am glad that I chose to stay and work it out. We have been married 20 years (plus I still loved him very much), so I didn't want to throw all those years away. Finally, whether you choose to stay or leave, you will have to also make the choice to forgive or not. I would choose forgiveness, more for your own sanity, then his. Just remember, what he did was loathsome, but he is human, and try not to define him by his one stupid action.
Anyway, just my humble opinion, and hope some of it helps you get through this initial period of shock. Good luck, and I will be thinking of ya.
PS You didn't say why he told you.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim