"I wish I could put into words how grateful I am for this man right here," she wrote on Instagram, alongside a photo of the two of them kissing.
"But my love has grown to a level that words could never possibly express how much this man completes me. 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," the 22-year-old singer continued, "he still never takes credit and I want the world to know how incredible his soul is. I really wouldn't be alive today without him. I love you Wilmer."
Hours before Lovato's Instagram post, Valderrama posted his own sweet note to the singer.
"Today, we lay another stone together," he wrote on Instagram. "It is now 3 years since you said: I am more important then any past, demon, disorder or addiction.. Where you decided that YOUR life was worth living while loving yourself.. And I'm sure glad you did... I knew then the woman you've become now.. I am proud and honor to be front row to see the world you are about to change."
The "Heart Attack" singer entered rehab in 2010, and she has been outspoken about her struggles with depression, bipolar disorder, an eating disorder and cutting in her pre-teen and teenage years.
Lovato also thanked her family, friends and fans for helping her reach this point.
But with the love and support I've been given from my friends, fans, and family.. I now celebrate my 3rd birthday. Thank you to everyone— Demi Lovato (@ddlovato) March 16, 2015
I'm still catching up to texts and congrats tweets... Thank everyone who has supported my journey. I never knew sobriety was possible for me— Demi Lovato (@ddlovato) March 16, 2015