I am so sorry that you are here and dealing with infidelity.
I can feel your pain as I read your post. I am sorry. You have been betrayed twice. Once by your husband and once by your friend. That is really hard to reconcile, absorb and understand.
You will never fully understand why the affair happened. Logically we want so bad to understand but affairs are never based on logic. The best I believe we can hope for is to understand how it happened.
Understanding the how can come through IC and MC. Your husband needs to figure out why he allowed himself to cheat. He needs to understand so he can assure you he will never hurt you again.
Your anger is totally understandable and completely normal. I think after a year most people are coming out of the shock and realize the spouse is still there so the fight/flight mode has minimized and BOOM - here comes the anger.
Angry that this happened, angry you are so hurt, angry that they chose to do this to you, angry that they are your biggest trigger, angry that they stripped away your self esteem and trust, angry they were selfish, angry at all the lies, angry...for everything.
Being angry is not wrong. It is a healthy and an appropriate emotional response to injustice. Anger is pain turned outward.
Anger serves a constructive purpose in bringing about healing. However, anger needs to be channeled in healthy and constructive ways.
IMO, a key to recovering from infidelity (especially if you are interested in rebuilding your marriage) is to avoid bad responses.
Make an anger list. Write down all the things that make you angry. Discuss them with your spouse, a friend or God. Discover ways to eliminate their sources, and then check them off. Just seeing some progress can help you.
It takes on average 2-5 years to truly heal from an affair and even then you will always have the scars. But an affair does not have to define you or your marriage.
No one that hasn't been through this can relate or understand the pain. So please allow yourself to feel whatever you need to feel. You are not obligated to work it out if you don't want to.
How is your husband now? Is he committed to helping you heal and remorseful?
Be strong and know that you can and will make it through either way.