Kate Middleton Serene in New Portrait as Prince William Says: ‘We Love You’

Prince William is celebrating his love and admiration for Kate Middleton as she marks her 43rd birthday.

In a message posted to the couple’s social media, William, 42, paid tribute to Kate after a year in which she had a major operation, a cancer diagnosis and debilitating chemotherapy treatment.

“To the most incredible wife and mother,” he wrote on Thursday, Jan. 9. “The strength you’ve shown over the last year has been remarkable. George, Charlotte, Louis and I are so proud of you. Happy Birthday, Catherine.”

“We love you. W,” he closed the message.

It was coupled with a stunning previously-unseen portrait of the princess, taken by Matt Porteous, in Windsor, last summer.

William and Kate don’t often post personal messages on their social media and this marked a rare public declaration of William’s love and support for his wife who has been through so much since her last birthday.

She is spending the day at home at Adelaide Cottage, Windsor with her husband Prince William and children Prince George, 11, Princess Charlotte, 9, and Prince Louis, 6, who returned to school after the Christmas break on Jan. 8.

Princess Kate was most recently seen on Christmas Day, as she and the royal family led by King Charles walked to and from church at Sandringham, Norfolk. She was in her element as she chatted with well-wishers, and many of her meaningful conversations with those gathered were about cancer. 

In March 2024, Princess Kate announced that she was undergoing treatment for cancer and shared in September that she completed chemo. In a rare revelation at her Together at Christmas carol service in December, Kate said, “I didn’t know this year was going to be the year that I’ve just had.”

Since announcing the end of her chemotherapy in a moving and intimate family video in September 2024, she has continued her gradual recovery. In November, Kate appeared at the Remembrance weekend honoring the fallen British and Commonwealth armed services members.

Robert Hardman, author of The Making of a King: Charles III and the Modern Monarchy, tells PEOPLE that the princess is still on the mend, however.

“We are being encouraged not to look at this as a chapter that’s over and then it’s back to normal, because cancer doesn’t work like that,” Hardman tells PEOPLE. “The details remain private but there’s no getting over the reality that it’s an unpredictable disease.”

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“The clue is in her phrase ‘one day at a time’ — that’s how she’s going to approach it,” adds Hardman, referring to a line in her Sept. 9 announcement about completing chemo.

The Princess of Wales said in that video message, “Doing what I can to stay cancer-free is now my focus. Although I have finished chemotherapy, my path to healing and full recovery is long and I must continue to take each day as it comes.”

Adds a source, “She has been allowed to get back in her own way. She has been able to pace it and do it her way, and that’s great.”