Jarrell Miller said he "messed up" and has apologised after a failed drugs test saw his June fight against Anthony Joshua called off.
Miller was due to meet British IBF, WBA and WBO heavyweight champion Joshua, 29, at Madison Square Garden on 1 June.
He was denied a licence for the New York bout after an "adverse finding" from a sample taken on 20 March.
Then after a second adverse finding was reported on Friday, the 30-year-old American admitted: "I made a bad call."
In a video message posted on Instagram, Miller said: "I messed up. There were a lot of ways to handle the situation, I handled it wrongly and I'm paying a price for it.
"I messed up a big opportunity and I'm hurting on the inside. My heart is bleeding right now - I hurt my family, my friends, my team, my supporters.
"But, I'm owning up to it. I'm going to deal with it. I'm going to correct it. I'm going to come back better and I'm humbled by the experience."
A day earlier, Miller had vehemently denied wrongdoing, insisting: "The facts will prevail and I shall be vindicated.
"My team and I stand for integrity, decency and honesty and we will fight this with everything we have."
That came after returning the initial adverse result from the 20 March sample and his team were awaiting the results of a test on a B sample for banned substances.
Then on Friday, Joshua's promoter Eddie Hearn said Miller had "failed a second separate test for a further substance".
Hearn said he had been informed of Miller's second adverse finding by the the Voluntary Anti-Doping Association (Vada), which operates within boxing and mixed martial arts.
The 20 March sample showed traces of a substance called GW1516, which is banned because it can improve aerobic power and endurance.
Hearn said a new opponent for Joshua will be announced next week.
The Madison Square Garden fight was set to be Joshua's US debut.
Among those under consideration to replace Miller are Cuba's Luis Ortiz, Poland's Adam Kownacki and American Michael Hunter.
Joshua is undefeated in 22 bouts and unified three of the heavyweight division's world titles by beating Wladimir Klitschko in the summer of 2017.