The former bantamweight champ tested positive for EPO -- a banned substance -- before his January title fight with Henry Cejudo. He's accepted a 2-year ban and won't be eligible to fight until Jan. 2021.

"I'm having a hard time trying to forgive myself for this," 33-year-old Dillashaw said ... "Which I SHOULD have a hard time."

"But, I got to man up to what I did."

Dillashaw initially played dumb about the positive test -- saying he would look into how the positive test came about.

But, now he's admitting he knowingly cheated -- and says he understands all of the criticism and scrutiny that comes his way.

Dillashaw says he vows to return to the sport and wants to prove to his 15-month-old son that he can come back after making a tremendous "mistake."

"This is who I am. I'm a fighter ... and I'm gonna have to fight my way through this one."

While he's on the bench, Dillashaw has already had surgery on one shoulder and will get surgery on the other soon.