I've felt what you're feeling before, I ache for you and know it's a terrible place to be in. For me, a lot of my negative, my-life-is-over thinking came from believing my H when he degraded our whole M. I couldn't stomach the thought that our M and years of my life were a cruel joke. I couldn't fathom trusting another person again.
But if you read enough on here, you'll see that most WAS say and do the same stuff--it's their justifying to themselves leaving: a marriage that really did mean something, a person that they loved, part of their family that they created through marriage. Leaving those things is so difficult that they actually have to rewrite history over the good memories in order to do it. And they are in so much pain themselves, they do reckless, terrible things to ease it--such as have affairs. In other words, don't believe that you never had a marriage, don't believe or take to heart what she says and does. You can take from your R with her what you decide to take, not what she says or does.
Focus on being a good man, do what you have to do to get out of patterns of negative, nihilistic thoughts. This is what worked for me--find what you enjoy doing or think about someone you care about that will interrupt those patterns. For example, think about your step-grandson, and how much he is missing you in his life right now. You said you two were best friends. Focus on how you can live a good life now and re-establish a relationship with him sometime in the future when things calm down a bit. And they will calm down.
Nothing is lost that you hold within you, like love for your GS. You have a whole new life on the horizon.
It is in the shelter of each other that people live.--Irish proverb
