Britney Spears and Sam Asghari Are Engaged: 'I Can't ... Believe It!'

Britney Spears and Sam Asghari are ready to say "I do!"

Spears and Asghari announced their engagement on Instagram on Sunday after more than four years of dating.

The singer and actor-fitness enthusiast were first romantically linked after meeting on the set of Spears' "Slumber Party" music video in 2016.

Spears, 39, and Asghari, 27, shared the happy news on Instagram, and Asghari's manager Brandon Cohen shared in an additional statement to PEOPLE that he is "proud to celebrate and confirm the engagement."

"The couple made their long-standing relationship official today and are deeply touched by the support, dedication and love expressed to them," said Cohen, adding that Spears' new ring was designed by N.Y.C. jeweler Roman Malayev, founder of Forever Diamonds in New York.

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"He couldn't be happier to be involved and make this one-of-a-kind ring," added Cohen.

In a sweet video, Spears — who a source tells PEOPLE exclusively is "ecstatic" about the proposal— showed off her new rock with Asghari by her side. "Yes!" she tells Asghari after he asks if she likes her new bling.

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The singer has had a year filled with up and downs as she continues to fight to end her 13-year conservatorship.

In August, Spears praised her now-fiancé on Instagram for helping her through the "hardest years" of her life.

"Not only has this cute asshole been with me through the hardest years of my life but he happens to be an extremely good cook 👨🏽‍🍳!" Spears wrote. "Fast & Furious franchise, don't miss out on your next star 🌟🏎️👗🤓 !!!!"

In a March interview with Forbes, the Iranian-born actor opened up about the beginning of his relationship with Spears and admitted he was ready to take the next step in his career — and relationship.

"My priorities in life are to remain humble and understand where I came from and where I'm going," he told Forbes at the time. "I want to take my career to the next step when it comes to acting. I want to take my relationship to the next step, as well. I don't mind becoming a father. I want to be a young dad."

A few months later, the singer made it clear as she gave her testimony during a conservatorship hearing that she wanted to marry and have a child with Asghari — and a friend of the couple told PEOPLE that he was also ready to settle down.

"He's ready for marriage and kids with Britney and is serious about the relationship. She wants to take that next step and have that full life with him," said the friend. "As a couple they've hidden how they've really felt even from people close to them because they were anxious and scared, but now it's all out there."

Following the explosive June court hearing, a source told PEOPLE that Asghari was the singer's "rock."

"Sam has been Britney's rock for years. He makes sure that she eats well and works out," the source said about Spears' boyfriend. "She leans on him for everything. He works and has his own life too, but is around as much as he can."

In an exclusive statement to PEOPLE following the release of an unauthorized Hulu documentary on Spears' life and conservatorship, Asghari said he was "looking forward" to a "normal, amazing future" with the pop star.

"I have always wanted nothing but the best for my better half, and will continue to support her following her dreams and creating the future she wants and deserves," he told PEOPLE at the time. "I am thankful for all of the love and support she is receiving from her fans all over the world, and I am looking forward to a normal, amazing future together."