Pistorius is the world record holder in three Paralympic events
Paralympic 400m star Oscar Pistorius finished fourth on his return to track action in Milan on Wednesday, his first able-bodied race for nearly a year.
It was also Pistorius's first race since being cleared to run against able-bodied athletes at the Olympics.
The South African double-amputee hopes to represent his country in Beijing but his time of 47.78 seconds was well outside the 45.55 qualifying standard.
"I'm disappointed with my time," admitted Pistorius.
I'm chasing something that may be unattainable, it's starting to look impossible
"I've only had a month and a half to train, but I didn't expect it to be this bad. I let myself down.
"I've got to be realistic. I'm chasing something that may be unattainable. It's starting to look impossible."
Pistorius only returned to training six weeks ago having won his appeal to the Court of Arbitration for Sport (Cas) in May that his prosthetic legs did not give him an unfair advantage.
The 21-year-old, whose personal best over the distance is 46.36, will have a final qualifying attempt in Lucerne, Switzerland on 16 July.
"I really have to pull up my socks if I am going to go to the Olympics," Pistorius said. "It is the last chance for me to qualify, and if I don't it will be my last able-bodied meet of the year."
The South African selectors have delayed naming their Olympic team until 17 July to give Pistorius every chance to make the team.
His best hope lies in the 4 x 400m relay squad, but he would still need to run close to 46 seconds flat, a significant improvement on his personal best.
"If I make the team I don't want to be the reserve for the relay, I want to be in the top four," he added. "I want to bring something to the race and make the relay stronger."