Hello SSL, you have just learned of your partners two year PA. Of course you are having outbursts of anger - even if you have been feeling unsure of your relationship (I read your first post) and have not been upfront about your marital fears (which by the way does not give him grounds to cheat to cheat), feeling angry is a step in this process. I had an outburst last night - we are 8 months in. It didn't last long but there it was.
I started this process - btw, my fwH had a two year PA with someone outside of where we live so like you, I knew something was up but could not place it simply bc he was home. Present - but not really - yway, I started the process by reading After the Affair by Janis A Spring. We both read it actually. The byline of the title refers to Healing the Pain and Rebuilding Trust. It's easy to read.
Since it sounds like you have had some long-standing concerns with marriage, opening up, I would suggest IC for both of you. It might help you weed through the things that have long troubled you. That way, whether you stay with him or not, you will have a better understanding of you.