Mod Sun Calls Avril Lavigne 'That Mother F---ing Princess' in Sweet Birthday Post: 'Forever Yours'

It's another year in the books for pop-punk princess Avril Lavigne — but regardless, "Here's to Never Growing Up!"

In honor of the special occasion on Tuesday, Lavigne's fiancé and fellow punk-rocker Mod Sun dedicated a sweet birthday post to mark the milestone.

"When I close my eyes it's you I see. You're the reason why my heart still beats. My love is yours eternally. I live inside each memory," he wrote, celebrating her 38th birthday. "Your eyes light up the darkest night. Your kiss inspires the words I write. Your touch is all I need in life. Someday soon you'll be my wife."

"You changed the world Avril never forget. You are that mother f---ing princess. You make me better I must confess. I'd whisper 'I love you' with my last breath," Mod Sun, 35, continued. "There's so many words I wanna say, To my stunningly gorgeous fiancée, I'll leave it at this til we runaway…. Baby I wish you a happy birthday."

He concluded, "To my soulmate, you are such a gift in this world + to say I'm lucky to share a life with you is the understatement of the century…not to mention you are also the most beautiful woman that's ever walked this earth 🥰 in two years together we have made so many memories but it's just the start. Let's runaway my love. Forever yours, mod."

The couple got engaged in March and shortly after, the "Complicated" singer spoke to PEOPLE about the Parisian proposal.

"It was the most perfect, romantic proposal I could ever ask for. We were in Paris on a boat in the Seine River. We had a violin player, champagne and roses. It felt like time stood still, and it was just the two of us in the moment," Lavigne told PEOPLE exclusively of the proposal.

And while Lavigne said the proposal "was a total surprise," she knew soon after they met that he was the one.

"I knew we had a very strong unstoppable connection from the first week of writing and working together in the studio. Immediately," said Lavigne, who met the "Karma" singer while working on her latest album, Love Sux. "He made every day better. It was like I had known him forever. As time went on, things kept getting better, and it felt incredibly natural."

The pop-punk princess had also recently opened up to PEOPLE about the origins of their relationship, revealing that she was not looking for love when Mod Sun (whose real name is Derek Smith) came along.

"I was like, 'I need a break from men,'" she said. "Then two days later that all went out the window!"

The pair began dating "immediately" after they met at a songwriting session, as Lavigne said she "just followed [her] heart."

RELATED VIDEO: Avril Lavigne Reveals She Was on "a Break from Men" Before She Met Boyfriend Mod Sun: "I Followed My Heart"

"I was getting out of a big relationship, and for the first time in my life, I thought I should probably be single," she said. "I got together with [Mod Sun] to write a song. We just clicked. I was like, 'Should you be doing this?' It was fun and exciting, and I decided, 'F--- it. Life's short.' I'm a Libra. I love love."

In April, the couple shared photos from their Paris-themed engagement party, where friend and collaborator Machine Gun Kelly was in attendance!

The event featured the soon-to-be bride and groom both wearing bright pink outfits and berets, with Mod Sun in a suit and Lavigne in a mini dress.

"Look how lucky we are," Mod Sun said to Lavigne in one Instagram video. "Look at all our friends! I love you all!"