Staff Sargeant Michelle Young, 34, took her own life, leaving behind her 12-year-old daughter, who friends described as “her whole world” in a heart-wrenching post on fundraising website GoFundMe.

“Michelle was a beautiful soul, an amazing friend, a single mother, a soldier, and is proof you never know what someone is going through or what demons they may be fighting,” friend Sarah Maine wrote on the page, reported the New York Post.

“We will never fill the gaps in our hearts and will always be left with questions,” she added.

Ms Young, from Prescott, Arizona, had served in the army for 16 years, first enlisting when she was about 18 years old. During that time, she completed two tours in Afghanistan, the Daily Mail reported.

Just days before news of her death broke, Ms Young had posted a birthday message on Instagram for her daughter, Gracie.

“Happy birthday to the sweetest girl I’ve ever known,” she wrote.

“The best part about my life is being your momma.”

Friends recalled how close Ms Young and Gracie were, with countless photos on social media showing a loving mother and daughter. Just this past Halloween, the pair dressed up as Travis Kelce and Taylor Swift.

Ms Young commanded a following of over 100,000 people on an Instagram account where she modelled apparel from a female veterans-focused athletic attire company Curves N Combatboots.

The influencer advocated for better mental health treatment for veterans while combating homelessness among former soldiers.

She also volunteered at her local shelter as a crisis trauma response volunteer.

In September, she made a post for Suicide Prevention Week recalling how her older brother took his life when she was 14 years old, and encouraged anybody in distress to reach out.

It remains unclear what led to Ms Young’s suicide.

“From the outside looking in it looked like she had it all,” Ms Maine wrote in a separate post. “Mental illness is so prevalent especially in the military. We need to find a way to do better.”

Nearly $US25,000 has been raised on Ms Young’s GoFundMe page. The funds will go directly to her daughter.

This story appeared in the New York Post and is reproduced with permission.