"I think it's safe to say that most of you know a lot of my life whether I liked it or not, and I had to stop, 'cause I had everything, and I was absolutely broken inside. And I kept it all together to where I never let you down. But I kept it too much together where I let myself down," said 24-year-old Gomez, who recently took time off to treat depression and anxiety.
"I don't want to see your bodies on Instagram. I want to see what's in here," she added, pointing to her heart. "And I have to say thank you so much to my fans because you guys are so damn loyal, and I don't know what I did to deserve you. If you are broken, you don't have to stay broken."
Grande was shaking onstage as she collected artist of the year at the fan-voted show that aired live on ABC, beating Rihanna, Carrie Underwood and Bieber. "With or without this, I love you from the bottom of my heart," she said to her fans.
Green Day was one of many big-named acts promoting new albums at the AMAs, including Bruno Mars, the Weeknd, Lady Gaga, Sting, John Legend and The Chainsmokers. But Green Day got political during their performance of "Bang Bang" as band leader Billie Joe Armstrong sang "No KKK, no fascist USA, no Trump" during the show.
The show's hosts — former Saturday Night Live comic Jay Pharaoh and model Gigi Hadid — also mentioned the Trump family when telling jokes: After Pharaoh impersonated Jay Z, Hadid acted like Melania Trump.
"I love my husband, President Barack Obama, and our children, Sasha and Malia," she said, scrunching up her face.
Strumming guitars while singing was one of the night's themes: One Direction's Niall Horan slowed things down for his acoustic-folk solo single, "This Town": James Bay sang his smoky single, "Let It Go"; and Shawn Mendes performed his hit "Treat You Better" and switched to piano for the aching "Mercy."
But the best singer-guitarist was Gaga, who beautifully belted the lyrics to her latest single, "Million Reasons." She earned a rousing applause from the audience.
Sting also impressed, with his performances of "Message In a Bottle," "Every Breathe You Take" and his new single, "I Can't Stop Thinking About You." He received the American Music Award of Merit, which was presented to him by Robert Downey Jr.
"American music lit the musical spark in my young mind," the English singer said after his performance. "My musical soul is all American!"
Other highlights included double winners twenty one pilots and Mars, who kicked off the AMAs with his groovy hit, "24K Magic," backed by his band who matched his slick, upbeat dance moves.
Grande and Nicki Minaj got steamy during their performance of the reggae-tinged "Side to Side": Grande's background dancers danced closely on her, and one male dancer even pulled up her skirt, while Minaj, who rapped from a chair, spread her legs widely.
Other performers included Maroon 5 with Kendrick Lamar; DJ Khaled with Minaj, Future, Rick Ross and August Alsina; and Fifth Harmony, who won collaboration of the year for "Work from Home" with Ty Dolla $ign.
Prince's 1984 Purple Rain album, which gained new attention after his death in April, won top soundtrack. His sister, Tyka Nelson, collected his trophy onstage and cried as she spoke to the audience.
"We'll keep this for you at Paisley Park until we see you again," she said weeping and naming her siblings. "This is for you."
Other winners included Tim McGraw, the Chainsmokers and Florida Georgia Line. Rihanna, who didn't attend, won three awards. Adele and Beyonce were also absent but both won an award.
Zayn, who has won a handful of AMAs with his One Direction bandmates, took home new artist of the year.
"Wow. This one just has my name on it, right?" he said. "This has been a crazy year ... I didn't expect anyone to vote for me, so thank you for voting for me."
List of 2016 AMA Winners:
Artist of the year: Ariana Grande
New artist of the year: Zayn
Collaboration of the year: "Work from Home," Fifth Harmony feat. Ty Dolla $ign
Tour of the year: Beyoncé
Video of the year: "Sorry," Justin Bieber
Pop/rock female artist: Selena Gomez
Pop/rock male artist: Justin Bieber
Pop/rock band, duo or group: Twenty One Pilots
Pop/rock song: "Love Yourself," Justin Bieber
Pop/rock album: Purpose, Justin Bieber
Country female artist: Carrie Underwood
Country male artist: Blake Shelton
Country band, duo or group: Florida Georgia Line
Country song: "Humble and Kind," Tim McGraw
Country album: Storyteller, Carrie Underwood
Rap/hip-hop artist: Drake
Rap/hip-hop song: "Hotline Bling," Drake
Rap/hip-hop album: Views, Drake
Soul/R&B female artist: Rihanna
Soul/R&B male artist: Chris Brown
Soul/R&B song: "Work," Rihanna feat. Drake
Soul/R&B album: "Anti," Rihanna
Latin artist: Enrique Iglesias
Alternative rock: Twenty One Pilots
Adult contemporary: Adele
Contemporary inspirational: Hillsong United
Electronic dance music: The Chainsmokers
Soundtrack: Purple Rain
Award of merit: Sting