"I wish I could put into words how grateful I am for this man right here," she wrote on Instagram, posting a black-and-white picture of herself and her beau smooching. "But my love has grown to a level that words could never possibly express how much this man completes me."
The onetime "Sonny With a Chance" star, 22, and the "That '70s Show" alum, 34, reportedly met in 2010 and reconnected in 2011, though their relationship status hasn't always been super clear. He declared his love for her last August in a birthday greeting on Instagram.
"He's loved me the way I never thought I deserved to be loved and with this day marking my 3rd year sober... After sharing my ups, putting up with my downs and supporting my recovery... he still never takes credit and I want the world to know how incredible his soul is," Lovato wrote Sunday.
"I really wouldn't be alive today without him," she said. "I love you Wilmer."
Lovato told "Access Hollywood" at the time that she "couldn't go 30 minutes to an hour" without using cocaine and would smuggle it onto airplanes and do it in the first-class bathroom.
Around 9 one morning, the "Heart Attack" singer said, she found herself drinking vodka out of a Sprite bottle and throwing up on the way to the airport -- where she was to board a plane to get back to a sober-living house where she was staying.
"I think at 19 years old, I had a moment where I was like, 'Oh my God… that is alcoholic behavior," she told AH. "[It's] no longer, I'm young and rebellious and out having fun, it was, wow, I'm one of those people."