She wasn't quite sure how it happened, but somewhere along the line she snapped.
It wasn't really Ron's fault. Well, it was and it wasn't. He was possessive, sure, and manipulative, in his own clueless way. He was prone to tantrums, fits, red-faced arguments over stupid, childish things. It never bothered her when they were adolescents, and she found his quirks endearing enough to marry him when they were still fresh-faced and young and stupid.
Eventually, though, Hermione grew up. Ron did not. He was still talking about quidditch and the Malfoys and how drunk he got at the pub last weekend even as Hermione accepted her grant money for advanced transfiguration research and registered herself as an animagus. Placating him became more difficult as her research demanded more and more of her time. They stopped having sex, and started fighting instead. They even began to take their meals separately. As bright as she was, it took her a long while to realize that there's only so much one can blame on extenuating circumstances and heavy work loads.
Her moment of clarity finally came when she stood on stage in front of her colleagues and peers, accepting the award those two years of research had earned her. Ron was there to support her, of course, even though he hadn't so much as said a word to her in the past three days. It was then, with her dress robes sparkling and her medal clasped firmly in her hand, that she realized that not only was it over but it had been for the past year.
Somewhere along the line Hermione just got tired. She grew weary of his arm draped across her shoulders, his stubby fingers twined with hers, his hard body shoved against hers in a clumsy fit of passion. She had come to loathe his harsh laugh, his temperamental attitude, even his ever-present scent of cheap cologne and sweat. She just didn't love him anymore.
She wasn't sure she ever loved him.
Obviously he wasn't pleased when she ended it, but Ron was never one to give up on anything easily. Still, it was relatively painless. She had refused to take his surname when they married (which, she realized belatedly, was something of a warning sign), and their prenuptial agreement was still considered satisfactory by both parties, so all they had to do is sign the papers and part ways.
What did it say about her when she watched four years of marriage spiral down the drain without shedding so much as a tear?
Last she had heard, he married Lavender Brown and the two of them had already spawned three children. Harry mentioned that parenthood agreed with Ron, and he had mellowed out considerably. Hermione didn't doubt that; after all, having three toddlers who couldn't yet sleep through the night was bound to take the edge off of any man. All in all, she wished them the best.
She still gets letters of condolence every once in a while, from old schoolmates and family friends who've heard the news in passing and feel like they need to offer her a shoulder to cry on. After all, they were the golden couple of the Wizarding World. Marching behind Harry, hand clasped, heads held high in the face of adversity and death, with a love that blossomed among all the horrors of war. The letters she gets are very somber and cloying, and suggest a multitude of coping mechanisms that she may need to draw on in her darkest hour.
Hermione likes to read them out loud and laugh. For between the unhurried teatimes/brainstorming sessions with Minerva McGonagall, the revolutionary concepts being unearthed in her own living room, and the exquisite feeling of waking up stretched across a king sized bed and not knowing where the day may take you, Hermione has never been more contented in her life.
Perhaps happy endings are just what you make of them.