Maria Sharapova says being a teenager with "a few hundred dollars" and "no sense of the future" was the toughest period of her career - not losing in the US Open last 16.
The five-time Grand Slam champion has not gone past the quarter-finals in her five major appearances since returning from a doping ban in 2017.
The Russian, 31, lost to Spain's Carla Suarez Navarro in New York on Monday.
"I think I've done plenty in my career," said Sharapova.
"[I've] established a lot for myself personally, professionally."
Sharapova became a household name when she won the 2004 Wimbledon title as a 17-year-old, going on to claim a career Grand Slam with victories at the Australian Open, French Open and US Open over the next 10 years.
The former world number one slid down the rankings after being given a 15-month doping ban for using meldonium.
She tested positive for the banned substance, which is used to control heart disease-related conditions, at the 2016 Australian Open, but has always denied cheating.
Since returning to Grand Slam action at last year's US Open, she has climbed to 22nd in the rankings and reached one quarter-final at Roland Garros in June.
Asked if Monday's defeat represented the most challenging period of her career, Sharapova - who left Russia for the United States with her father as a seven-year-old - said: "What's challenging is when you're a teenager and you have a few hundred dollars and you've got no sense of the future.
"You don't know where you're going to end up. You just have a dream.
"That's a lot tougher than being 31 years old and having the opportunity to do whatever I want in my life."
After losing to 30th seed Suarez Navarro 6-4 6-3, Sharapova added she would not still be playing if she did not believe she could challenge for Grand Slam titles again.
"If you don't have belief, it's your choice to not continue that," she said.
"If I didn't have the belief to keep doing this and to keep having the motivation and the grind of doing this every day in order to get myself in these positions, I don't think I would be here.
"The belief is not something that I'm eager to show everybody else.
"The belief matters most when it's internal and when you have a passion for something."