But I feel so low and destroyed by what happened, I just feel like giving up
It's normal to feel that way, even though we don't want to and try our best to figure out a way to climb out of that pit. You are doing the right thing, my friend, by coming here. This place has some of the best people who are willing to go above and beyond to help.
When I was snooping after my ex's almost year long PA went public, I found other evidence dating back to 2010. Note that we were married in 2008. Two other EAs - not sure if they went PA, but they could have when she traveled for work - with ex bf's. Nothing like coming across an old phone and finding messages/VMs that weren't erased. I didn't even bother confronting because I thought "what's the point?"
It's been almost 2 years since the BD was dropped and 1 year since I got proof of the PA - which, coincidentally, started BEFORE the BD. What has helped me? Time. And my kids. That's it. If you don't already have one, see an IC. I did - one who was our MC - and it helped me tremendously.
People talk on here of GAL - which seems like the furthest thing from your mind now, but trust me - it works. As hard as it is, it works.
There are moments in this life when you are so confident in the rightness of your actions, that not even for a second do you consider the option that you might be wrong.