Jarrell Miller accepted he "messed up" after three failed drugs tests cost him a title shot against Anthony Joshua.
Heavyweight champion Joshua was scheduled to defend his WBA, IBF, IBO and WBO titles against the American at Madison Square Garden on June 1.
However, promoter Eddie Hearn revealed Miller had failed two drugs tests, leaving him to find a new opponent for Joshua. And then it was revealed that Miller failed a third drug test - with banned performance enhancing drugs like EPO and HGH (human growth hormone) coming up in his tested samples.
Miller initially denied ever knowingly using a banned substance and confirmed he would appeal the New York State Athletic Commission's refusal to give him a boxing licence.
But in a video posted on Instagram on Friday, the 30-year-old said he had made a mistake.
"This is your boy 'Big Baby' Miller here. A lot can be said right now, I'm going to get straight to the point," Miller said.
"I messed up, I messed up, I made a bad call. A lot of ways to handle the situation, I handled it wrongly and I'm paying the price for it. Missed out on 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 to understand how to handle certain things and I'm going to leave it at that.
"I love you guys and I appreciate you guys out there. Fighters, we go through a lot and I don't want to make a bad name for ourselves and it's time to do right and get right."