NASA astronaut Kjell Lindgren learned a trip into space would be possible watching the first space shuttle launch in second grade, but in 1998, he faced what he believed was the obliteration of his lifelong dream.

While in Air Force flight training, he went to the flight surgeon with what he thought was a cold, it turned into an asthma diagnosis, and he was pulled from pilot school.

Months went by as he waited for a decision on whether his pilot training could continue and Lindgren, during that time, started night classes to prepare for the medical college admission test and applying to medical schools. He wanted to stay in the Air Force, even if he could not fly, and pursue medicine.

But to his surprise, not only was he told he could not fly, he was medically discharged from the Air Force. An appeal board later upheld the decision.

“I kind of likened it to descending into a valley of despair,” he said, during the Air Force Academy’s National Character and Leadership Symposium on Thursday.

He watched his good friends get their wings and select their aircraft in what he described as a bittersweet moment. The next day, the Air Force Academy graduate received his acceptance into medical school.

It was a pivot driven by a dark day, “maybe even a cursed day,” that he now recognizes as a blessing, he said.

“I can tell you with almost 100% certainty that had I not been kicked out of the Air Force, I would not be here today,” he said.

He went on from medical school in Colorado to train in emergency medicine in Minneapolis where he had the opportunity to care for patients in a hyperbaric chamber, where patients breathe pure oxygen in a pressurized environment. An asthma diagnosis could have disqualified him from this work, as well. But when he was tested for asthma in Minnesota, the doctors didn’t find the disease, opening the door for him to pursue spaceflight once again.

So, after his training in Minneapolis ended, he went to Galveston, Texas, where he pursued a second residency in aerospace medicine. From there, he went on to work as a flight surgeon at NASA and in 2009, he was accepted into NASA’s 20th astronaut class. He has since logged 311 days in space.

On his first trip to space in 2015, seeing the sun rise over the curve of the Earth in space, it was the realization of a lifelong dream and granted him greater perspective on his discharge from the Air Force.

“Many of us go through incredible hardship and challenge and don’t get to see the bright side. … We lack the context. We lack the perspective,” he said.

He encouraged the crowd, including many cadets, to find their own inner motivation to help them move through dark times. For him, it was a desire to serve that helped him transition from the Air Force into medicine.

“Don’t let others define what is possible. Let your hard work create opportunities and open doors,” he said.