Speaking to Access Hollywood, the actor - who recently confirmed that he has "beat" the disease - said that he still feels frustrated about the length of time it took to identify the illness.
Douglas explained: "I went and saw a round of doctors in January to February and they didn’t pick up anything. They gave me some antibiotics and then I went and saw a round of doctors again in May and still nothing.
"Then my tongue started bothering me and by the time I took off for the summer and came back in August, I told my doctor ‘Hey, something’s going on'."
He added: "I understand it’s a very difficult cancer tumour to find. It was under the tongue and it’s unusual, but I’m still doing my homework on it.
"I would say I have not forgiven. Stage two would have been nicer than stage four."