Jason Segel and Michelle Williams were so cute and low key together ... and now they're done, and we're sad.

Us Weekly reports that Williams, 32, and Segel, 33, simply couldn't make their long distance romance work.

Williams resides in New York City with her seven-year-old daughter from Heath Ledger, Matilda, while Segel stays in Los Angeles where 'How I Met Your Mother' films.

The pair were first spotted together in March 2012, and all signs pointed to both being in it for the long haul: neither commented publicly on the romance, and Segel bonded with Matilda.

"He's been doing all sorts of 'dad stuff' with her, like drawing and playing music," a source said, adding that Segel was particularly doting of his short-haired lady. "[They're] totally in love. He's obsessed with Michelle."

The couple were spotted on vacation together as recently as January, but their schedules didn't allow it to stick. On the bright side, 'HIMYM' only has one more season left -- so it may not really be the end for these two.

We hope. Seriously, between them, Coco and Ice-T and Danny DeVito and Rhea Perlman, they need to make this work for the sake of our belief in love.

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