they're happy with the way things are right now
THAT my dear is pure conjecture on your part. you spend a lot of time with these two do you? you speak to them individually about their FEELINGS?
i think you might need to give definition to this HAPPY thing.
and also ACCEPT that some people do not belong together, they come from different puzzle boxes and no matter how turn them or force them, they are NEVER going to fit. and that is ok, it just didn't work out, it just wasn't meant to be a long term thing. i was with my last husband for 14 years....we had some good times, he was always a great guy, but we just weren't long-lived. we split up and moved on, each in our own way.
he didn't FAIL me. i just no longer BELONGED with him. it was time to go.
you didn't fail your ex. it just became apparent that you two were no longer compatible. different wants and needs. and thus you are both now free to seek that which is compatible and conducive to your true happiness.