The three-time NASCAR Sprint Cup Series champion hit and killed fellow driver Kevin Ward Jr in an incident during a dirt-track race on August 9.
Stewart, 43, returns to racing for the first time as he prepares to compete at the Oral-B USA 500 on Sunday.
In a statement facing a news conference on Friday, Stewart described the pain he had gone through.
"This has been one of the toughest tragedies I've ever had to deal with both professionally and personally," he said.
"This is something that will definitely affect my life forever. This is a sadness and a pain that I hope no one ever has to experience in their life.
"With that being said, I know that the pain and the mourning that Kevin Ward's family and friends are experiencing is something that I can't possibly imagine.
"I want Kevin's father, Kevin Sr., and his mother Pam, and his sisters Christi, Kayla, Katelyn, to know that every day I'm thinking about them and praying for them."
Stewart thanked his friends and family, as well as the NASCAR community and his team, Stewart-Haas Racing.
He took no questions, wanting to respect the ongoing investigation, but said racing would help him move forward.
"I've taken the last couple weeks off out of respect for Kevin and his family, and also to cope with the accident in my own way," Stewart said.
"It's given me the time to think about life and how easy it is to take it for granted.
"I miss my team, my team-mates. I miss being back in the race car. I think being back in the car this week with my racing family will help me get through this difficult time."