Jagger posted the message on his Facebook page as the Rolling Stones canceled their seven-date tour of Australia and New Zealand, "14 on Fire," in the wake of Scott's death. The noted fashion designer was found dead Monday in her Manhattan apartment, an apparent suicide.
A report Tuesday in the New York Times shed possible light on Scott's state of mind in recent days. Former Times fashion writer Cathy Horyn, a friend of Scott's, wrote in an essay posted on the paper's website that the designer had been planning to announce on Wednesday that she was closing her business, apparently due to financial troubles. Horyn wrote that she had learned of that development after Scott's death—and did not say from whom.
On Facebook, Jagger wrote: "I am still struggling to understand how my lover and best friend could end her life in this tragic way. We spent many wonderful years together and had made a great life for ourselves. She had great presence and her talent was much admired, not least by me."
He added: "I have been touched by the tributes that people have paid to her, and also the personal messages of support that I have received.
"I will never forget her."
Jagger was believed to be on his way to New York after hearing the news in Australia. The band issued a statement expressing regret that the tour was being postponed.
Meanwhile, details emerged in Britain about the financial troubles that Scott's eponymous label had been experiencing. Last month the designer canceled her London Fashion Week show, citing production delays.
Accounts filed by Scott's LS Fashion Ltd. in London show the company had liabilities that exceeded assets by 4.24 million euros ($5.9 million) as of Dec. 31, 2012.
The company's long and short-term debts totaled 6.75 million euros against assets, capital and reserves of 2.51 million euros, according to the accounts, which were filed in October.
Scott's high-end fashion label was a favorite among Hollywood stars like Nicole Kidman, Olivia Wilde, Tina Fey, Sarah Jessica Parker and others, not to mention first lady Michelle Obama. Recently she collaborated with Banana Republic on a more affordable line of clothing.
Scott was adopted by a Mormon couple and grew up in small-town Utah. She made her way to Paris after high school where, aided by her 6-foot-3 height (some say it was 6-foot-4) and striking looks, she found work as a model for some prominent photographers.
But she became more interested in working with clothes than modeling them, and eventually made her name as a top stylist in Los Angeles and a costume designer for films including "Ocean's 13."
In 2006, Scott founded her label, with an initial collection based on the "Little Black Dress." She became known for designs that used luxurious fabrics and had a vintage, old-fashioned feel, such as her famous "headmistress" dress—prim, with three-quarter sleeves, but also close-fitting and stylish.
Lawless reported from London. Mesfin Fekadu in New York and Thomas Adamson in Paris also contributed to this report.