He made his choices, because he is an autonomous adult - if he did let anything about his relationship with you sway his decision to end it with her, that was still his choice. You didn't manipulate him or steer him to that. So, stop blaming yourself. It's done now, and his ex will deal with it as she sees fit - who knows, she might have wanted to end it, too, and could be feeling relief right now.
Indulging in feeling guilty serves no purpose - guilt is a useless emotion. Make an effort to be aware of those moments when your thoughts pull you out of the present and into dwelling on guilty feelings, because that's just a mind-fuck.
The boat just hit some choppy waters, that's all. You got tossed a bit by the unexpected awkwardness of it happening before your eyes. It's okay to feel weirded out a little, but don't attach yourself to the discomfort. Let it go.
When you start spiraling into those thoughts that bring up guilt and feelings of responsibility over his other relationship (which is exactly NOT your responsibility), do something physical to snap yourself out of that and get back into your body and in the here and now, whether it is taking a walk or snapping a rubber band. Get out of your head and stop ruminating over it.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein