although I know her cheating is just a symptom of her MLC, like how could she do this? It's just not her? It's all just so unbelievable to me
Sometimes it is a symptom of a MLC and sometimes it isn't. This could have been coming for a long time. Maybe yours has something deeper like mine did, which wasn't a MLC. Whether it is or isn't is irrelevant as it won't change the inevitable outcome. Don't concentrate on WHY she cheated, as it will only drive you crazy. Ask me how I know. You won't ever find the reason, no matter how hard you look or what you read. Sure, some do it because of a MLC...and some do it simply because they don't care. Nothing more, nothing less.
This is your time now. I have found that the gym heals all sorts of things. Exercise is the best antidepressant there is. Godspeed, my friend.
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.