Payne, 29, was reunited with his boyband co-star on Thursday when he attended the premiere for Tomlinson's new documentary All Of Those Voices.
The singer was seen standing up and applauding Tomlinson, 32, from the audience at the movie theatre in London's Leicester Square, with the pair later seen catching up following the screening.
Watch: Louis Tomlinson addresses his struggles with fame
The reunion then prompted a heartfelt and candid post on Instagram from Payne as he aired his anguish over feeling "ashamed" at having failed to be there for his friend during his journey professionally and personally.
Payne also thanked Tomlinson for supporting him through "something so dark I’ve never shared it," as he heaped praise on him admitting viewing the 90-minute movie had left him feeling "emotionally drained."
He wrote: "Seeing the world through your eyes last night was the most beautiful thing to experience.”
“My neck hurts from how much I'm looking up to you right now you were already my friend and brother but getting to look through that window into your world and mind just extends that respect I have for you.
“What you’ve dealt with and how you held it all inside!! I’m so sorry I was so out of my mind and I didn’t do better for you, I feel ashamed in those moments to not be as good of a friend as you have been to me, at least I have time now and I’m me again so will try and make amends.”
Payne then continued to admit his admiration for Tomlinson for showing so much strength throughout his difficulties - which included the tragic loss of his mother Johannah Deakin who died in 2016 aged just 42 after battling leukemia.
He continued: “I’m emotionally drained watching the movie and I only did an hour and a half, you lived it and I’m so proud of how you did it not only with amazing music but with passion and the will to get up off the canvas every time life beat you down and fight back.
“I’m so thankful to have you in my life mate I feel so lucky and I know you know this but you were part of a small group of people that literally saved my life, you dragged me out of something so dark I’ve never shared it but your approach is something I admire and aspire to so I can one day do the same.”
Payne then urged his fans to go and see the documentary as he credited Tomlinson for his hard work admitting "you deserve all these moments," as he credited a "very special man."
“Thank you Louis and congratulations you deserve all these moments, every sight you get to see, every cheer that you hear and all the love you are receiving from the world.
“You worked harder than any of us to get here and now the world gets to see that from your side… guys go watch the movie, please he deserves it and you’ll learn a lot about yourself from this very special man @louist91.”
All Of Those Voices is in cinemas for a limited time from 22 March.