February 09, 2016 03:50 PM

Prince Harry‘s hero, U.S. Marine Kirstie Ennis, is making major strides in her recovery following a second amputation in December.

On February 3, the Afghanistan veteran received a new prosthetic left leg. And by Tuesday she was already walking unaided – a feat she shared in exclusive video with PEOPLE, calling the milestone “super exciting.”

“When I saw myself stand for the first time, I couldn’t help but cry,” Ennis, 25, tells PEOPLE. “I just stared in the mirror. It’s the first step to getting my independence back.”

Ennis, who shared an emotional hug with fellow veteran Harry, 31, at Buckingham Palace in November at the finish of a 1,000-mile Walking with the Wounded trek around the U.K., is steadily learning to adapt to her new limb.

“We take for granted what seems like the simplicity of walking,” she says. “I’m like a toddler learning everything all over again. I have to think through a whole process in order to get the prosthetic to do what I want it to.”

The Pensacola, Fla., native, who suffered multiple injuries in a helicopter crash in 2012, has faced some 39 operations since then, including her first amputation in November. Then, following complications over the holidays, she was forced to undergo a second amputation above the knee.

“You don’t realize how strong you are,” she told PEOPLE at the time.

Since receiving her new limb at San Diego’s Naval Medical Center on February 3, “the prosthetic tricks my mind into thinking that my leg is there, which helps reduce some nerve pain,” she says. “The pressure and weight on my residual limb feels great too. No weight really goes onto the end of my leg. The suction keeps everything secure and distributes the weight through my pelvic cradle.”

The determined athlete, who has already resumed training to compete in Harry’s Invictus Games for wounded warriors in Orlando this May, admits, “I would get frustrated if I didn’t kick appropriately to step, but it’s all a learning process.”

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Now Ennis, who has been supported by the Bob Woodruff Foundation, and her medical team must ensure that the limb is a perfect fit long-term. That decision is likely to be made this week.

“I can’t just take a leg home that I don’t know how to use,” she explains. “They are very intricate pieces of technology. For example if I fall I have to know how to get back up.”

Getting back up is what she has had to do countless times since June 2012, but her motivation remains undiminished.

“I will do physically therapy and learn the basics like walking unassisted, getting up if I’ve fallen, getting in and out of chairs, and so on. It’s lot of strength training to build up the muscle I’ve lost.”

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