Religion forum, might be better suited for this, maybe...
The whole aspect of marriage, these days isn't what it used to be. Not trying to diss people who believe in it still.
But facts show, most of them fail. And... well I wont even go further into it all.
Humans are growing faster and faster. Tech, science, etc. help our children to become smarter much earlier than before. So Imo. ppl shouldn't worry about "oh but we have kids, we have to get married or stayed married, for the kids!"
They aren't dumb, they get it and will sooner than later accept and move on from such things.
As for the partner feelings. Everyone from time to time feels such emotions. Depending on how you deal with them is the key.
And there's nothing wrong with 2 ppl just growing apart from one another and moving on to different things in life.
It's a solo journey Imo. Need to live it out the best way it fits you, and not the rest of the world. (<-- a very small amount of ppl believe that to be wrong, they believe we are all connected and it's a journey that we're all in together, I'm not dissing them, just trying to include different sides of it all)