The `Sex and the City` star - who has never been married - began dating the `Newsroom` writer in May but their relationship has cooled already because their lives are too busy with other things.
A source told New York Daily News newspaper: "They`re very different people. She`s focused on raising her daughter, Gemma, and he`s working on his show. They`ll definitely remain friends."
The 47-year-old actress - who adopted baby Gemma Rose in October 2011 - and Aaron made their first public appearance as a couple when they appeared on the red carpet for his new show `The Newsroom` in June.
Kristin is currently starring on Broadway in `The Best Man` alongside Cybill Shepherd and she has previously admitted the job is perfect as it allows her to spend a lot of time with her daughter.
She said: "The show is at night most of the days, so I spend all morning and all day with her then I go do the show at night.
"It`s actually a really great schedule for a mum. I`m able to be with her and have a job, which is rare and wonderful."