Rose Sandler was smart as a whip. She had wide interests, ranging from canoeing, to scuba diving, to crossing Mongolia’s Gobi Desert by camel.

As a Penn State graduate specializing in materials science, she was much in demand. One company hired her to work in California, and even paid for her to move there.

But her time there was spoiled when depression set in, undermining her confidence and ability to function. Despite the coronavirus pandemic, her parents, Lisa and Jay Sandler, flew out west in July, and brought Ms. Sandler back home to Bucks County. It would be her final stop.

Overcome by dark thoughts, Ms. Sandler took her own life Monday, Nov. 23, at home in Pipersville. She was 39.

Her family said they went public with Ms. Sandler’s struggle to try to help others wrestling with depression and other mental illnesses during the COVID-19 pandemic. Calls to suicide hotlines have jumped because of increased loneliness and hopelessness, experts said.

“She just couldn’t get herself out of the depression,” her mother said. “We pursued every kind of therapy, and she cooperated as best she could. We’re in utter despair for the time being. We’re doing everything we can to keep ourselves going.”

Ms. Sandler struggled with mental illness her whole life, her mother said. She would thrive for several years, and then depression would loom. “Each time it came back it was worse,” her mother said.

Diagnosed with attention deficit, bipolar, and borderline personality disorders, she sought in-depth treatment in Poway, San Diego County, where she lived for three years, but it wasn’t enough.

Her therapist, Michele Manker, said Ms. Sandler had the most severe case of depression she’d ever treated: “We still can’t fix certain things in certain people. It’s still such guesswork. We would like to think of love as a fix, but it’s not.”

Her mother blames herself for not paying closer attention. “If I could do it over again, I would ask her every day: ‘How are you, what are you feeling?’” her mother said. “Don’t just coast along thinking everything is all right. I feel like I didn’t tell her enough how much I loved her, how much she meant to me.

“I should have asked: ‘Are you thinking about taking your own life today?’ She promised to tell me and she didn’t.”

Born in Doylestown, Ms. Sandler grew up in Pipersville. She attended Central Bucks East High School through junior year, when she refused to go back to school. “Her teachers would say to us, ‘How do you get her to do anything?’ because she didn’t care about consequences,” her mother said. She did eventually earn a high school diploma.

Ms. Sandler was an accomplished horse rider. One early job was assisting a woman in Massachusetts who ran an equestrian center. From there, she moved back to Pennsylvania, graduating from Penn State with a bachelor’s degree in special engineering.

“I glue stuff together and then I break it,” she told her parents in a wry description of her occupation.

She went to work for Sikorsky Aircraft Corp. in Stratford, Conn., where she hoped to become a helicopter mechanic. “She just loved it for a while and then something happened,” her mother said. “She said, ‘I want to drop this and move on.’”

While in Stratford, she met and married Cameron Byrd, a computer engineer. The couple divorced after three years.

She began work on a master’s degree in Connecticut, but didn’t finish. Instead, she moved to North Carolina to work at and manage a 40-horse stable.

Three years ago, General Atomics, an aerospace company in San Diego, hired her and moved her to Poway. “I think I’ve met my match,” she told her parents.

“She got a great job there, and it was fine for two years,” her mother said. “And then something gets into her, it gets crazy. She started doubting her abilities. She started to fall apart. We went out there many times to encourage her, and get her set up with help.”

On her good days. Ms. Sandler was exuberant, fearless, and open to adventures. She went scuba diving with her dad in the Bahamas, and rode camels in the Gobi Desert during the Mongolian winter. She paddled in San Diego with the Hanohano Outrigger Canoe Club.

In the last four months, she cooked with her father. “The saddest part was, she had the world on a plate, but she lost the ability to enjoy anything,” he said.

Besides her parents, she is survived by a sister, Jenny.

Memorial donations may be made to Traveling Stories, a nonprofit dedicated to improving the reading skills of children via travelingstories.com, or to any food pantry. Ms. Sandler volunteered for Traveling Stories.

Anyone in distress or facing a mental-health crisis can call the National Suicide Prevention Lifeline at 1-800-273-8255. The line is answered 24 hours a day.