Wonder Woman Star Lynda Carter Is Still Struggling with Grief 3 Years After Husband’s Shocking Death (Exclusive)

For Lynda Carter, the pain over losing her husband Robert Altman to a rare blood cancer in 2021 will never truly go away.

"He was the great love of my life," she tells PEOPLE of Altman, a lawyer and video game executive whom she married in 1984 and raised two children with. "I think of him every day. There's so much of my day to day where I find myself thinking, 'Oh Robert will know.' Or if I'm on a flight, I still expect him to text to ask, "Did you land safely? Was everything alright?'"

Carter, 72, is now honoring Altman with a song she wrote for him called "Letters From Earth." She says people are still surprised to find out she's a singer, though she was actually doing it before she became known as the lead of Wonder Woman when the show launched in 1976.

"I've been singing for over 50 years," she says with a laugh. "I started singing in clubs when I was 14, and was writing music before I became an actor."

"Letters From Earth," she says, is a love letter to those we've lost.

"The soul of the song is about how you miss a person so much, because they're such a presence in your life that it's almost impossible to imagine that you won't physically see them again in this life," she explains. "You wonder, 'How can I communicate with you? Where are you? You didn't just die — there's too much of you left on this earth, too many people that love you.'"

She adds, "These letters are almost a prayer to your loved one. Hopefully people will relate to it. It's a celebration of love, as much as it is heartbreaking."

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Since Altman's death, Carter has partnered with City of Hope to help fund critical research complicated blood cancers like the one Altman died from. He was first diagnosed with myelofibrosis, a malfunction of cells in the bone marrow, which transformed into secondary myeloid leukemia, for which treatment options are limited.

"I'm so excited about all the advancements that have been made," she tells PEOPLE of recent scientific breakthroughs.

Carter says that while writing music, spending time with family and doing charitable work has kept her busy, she still deeply misses having a life partner.

"My life was an adventure with him," she says. "We were always traveling and doing fun things, and then when you're alone, you have to do everything alone. And that's no fun."

She adds, "Of course I've got some very good girlfriends. And the children [daughter Jessica, 33 and son James, 35] are wonderful. They also had a wonderful relationship with him, and we talk about him all the time."

She says her love with her husband worked because Altman "got a kick out me."

"He was this very calm, great, kind of an even-keeled person, and he liked my personality and the things I'd come up with. It was an interesting life together," she recalls. "We did fun things and we laughed and laughed. We had children, wonderful friends. We went through some serious things but life was an adventure with him."

She adds, "Of course, the memory of them keeps your love alive. But it's still a mystery when you look up to the sky and wonder, 'Where did you go?'"

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