Caitlyn Jenner reflects on Khloe Kardashian's birthday: 'I know I haven't been perfect'

June 28 (UPI) -- Caitlyn Jenner was feeling reflective on her former stepdaughter Khloe Kardashian's 39th birthday.

Jenner, 73, marked the occasion Tuesday by posting an emotional tribute to Kardashian on Instagram.

Kardashian and her siblings, Kourtney Kardashian, Kim Kardashian and Rob Kardashian, are the children of Kris Jenner and the late Robert Kardashian. Jenner was stepparent to the foursome during her marriage to Kris Jenner from 1991 to 2015.

Jenner, an Olympic gold medalist and television personality who came out as a transgender woman in 2015, shared a photo Tuesday on Khloe Kardashian's birthday. The picture shows Jenner and Kardashian at Kardashian's 2009 wedding before Jenner's transition.

"Happy birthday @khloekardashian Being your stepfather was, and is, one of the best things that I was ever blessed with by your mother. I promised your father that I would do my best to always be there for you kids and I know I haven't been perfect but I will always love you and you are forever in my heart," Jenner captioned the post. "I hope your day is filled with love, your kids, and happiness."

Jenner has six children of her own: Kendall Jenner and Kylie Jenner with Kris Jenner, Brandon Jenner and Brody Jenner with Linda Thompson, and Burt Jenner and Cassandra Marino with Chrystie Scott.

Jenner has had a strained relationship with the Kardashian siblings since her split from Kris Jenner in 2014.

Kris Jenner marked Khloe Kardashian's birthday Tuesday by sharing a compilation video of home movie footage.

"Happy birthday to my beautiful bunny @khloekardashian!!!! Khloe you are the sunshine of my life... You are the most amazing daughter, mommy, sister, auntie, best friend, neighbor, and such a huge part of my heart!!!!!" she wrote.

Kim Kardashian also posted a tribute to Khloe Kardashian on Instagram.

"Happy Birthday to one of my favorite human beings @khloekardashian Koko, you really are the definition of love, strength, and light! You bring life to any room you walk in! You protect your squad like none other. I'm so beyond lucky you're my sister and I just couldn't imagine adulting and parenting and just life without you!" she captioned the post.