"I had the ring in my pocket for two weeks," the Backstreet Boy tells PEOPLE exclusively of his engagement to fitness expert/actress Lauren Kitt.
"I was like, 'I don't want it anymore!' It was driving me insane. I just wanted to get it out of my hands and on to hers."
For the momentous occasion, Carter, 33, took his fiancée, 29, to the Florida Keys where he planned a romantic boat ride to a secluded island – but things did not go as smoothly as expected.
"My boat was in such bad condition because I hadn't used it for, like, six years. It took me five days literally on my hands and knees, scraping up my body [to fix it,]" Carter says. "The engine wasn't complete yet but we still decided to go out."
Despite the engine stalling a few times as they set out to sea Wednesday, Carter and Kitt – along with some close friends and Kitt's father – arrived at a small island Carter had spent time on as a child.
It was there, on the island the couple now refers to as "Engagement Island," that the Backstreet Boy dropped to one knee and presented his girlfriend of over four years with a seven-carat diamond ring from XIV KARATS.
"She said, 'Yes,' and I'm, like, 'I don't know what to do," Carter tells PEOPLE of the proposal. "She said, 'You're supposed to put it on my finger.' So I did. I was in shock."
The proposal came as a surprise to the bride-to-be as well.
"I wasn't expecting it. It just felt very surreal," explains Kitt. "We both are just so happy in love right now and on cloud nine."
And while the couple is excited for their wedding, there are a few other people they must consult before planning their impending nuptials – the band.
"Our first thought was, 'We need to contact the Backstreet Boys and find out when their schedule will permit,'" Kitt says of wedding planning while the band works on a new album and prepares for their sold-out fan cruise in October to celebrate their 20th anniversary.
"It's not up to us. It's up to the schedule. We're just excited and looking forward to the next chapter of our lives."