Mixter announced the news with a photo of the former flames and an emotional Instagram caption on Thursday, April 29.
"This is a photo of two best friends. This guy and I have shared so much life together. Changed each other at a soul deep level. All for the better," the 42-year-old began.
"The depth of our bond was something I could never have anticipated. I’ve never spent so much time with another person, grown so much alongside someone else, laughed so much, cried so much, shared so much. It is odd to write about the end of a relationship, especially when the friendship that still exists is so full of life. But change happens and we made the decision quite a while ago to let ourselves grow away from what our relationship had been and into what it was becoming," she continued.
The artist, who is normally a "private" person, went on to say that detailing the end of a public relationship is "like navigating a road that doesn’t even exist."
She continued, "My feelings while writing this are only ones of love and gratitude. Gratitude to the universe for putting this man in my life the way it did and then allowing us the grace to decide that what we had, could become more. Because that is what our friendship feels like. More. More than either of us could have asked for in another person.
"This post is a tribute to love and friendship and those relationships that make life worth living. Thank you J for being my best friend. The person who shares all my favorite jokes, sees who I really am and all I want to be, loves me unconditionally and makes me always strive to be bigger and better than I was yesterday. I will never stop cheering you on with every cell of my body. I will never stop loving you unconditionally," Mixter concluded.
Fans and friends took to the comments to offer their support to Mixter. "Sending you lots of love! Not an easy thing to go through ❤️," one user wrote.
"Love to you both on this new journey ❤️❤️ thank you for sharing," another said, while a third wrote: "Wow. Sorry to hear you have split, but that was one of the loveliest things I’ve read in a very long time. Sending you both Love and happiness for your futures."
Segel has yet to comment publicly on the split.
The pair began dating in late 2013 but kept their relationship under wraps and only made their red carpet debut at a fundraiser held by Sean Penn in 2016.