Probably the main stumbling block for people to see this truth of eternal destrutction (annihilation) is Gods perception of time is not ours.
Forever in Hebrew means something like "to the horizon", like as far as the eye can see.
Jonah the Prophet confessed he went to Hell forever!
2:1 Then Jonah prayed unto the LORD his God out of the fish's belly,
2:2 And said, I cried by reason of mine affliction unto the LORD, and he heard me;
out of the belly of hell cried I, and thou heardest my voice.
2:3 For thou hadst cast me into the deep, in the midst of the seas; and the floods compassed me about
: all thy billows and thy waves passed over me.
2:4 Then I said, I am cast out of thy sight; yet I will look again toward thy holy temple.
2:5 The waters compassed me about, even to the soul: the depth closed me round about, the weeds were wrapped about my head.
2:6 I went down to the bottoms of the mountains;
the earth with her bars was about me for ever: yet hast thou brought up my life from corruption, O LORD my God.
Jonah 2:1-6
Yet Jesus telling the same story said he was there 3 days.
12:40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. Matt. 12:40
The difference is in this.
3:8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
2 Peter 3:8
Between God and Jonah those 3 days were like forever! Like they would never end. Yet Jesus (who put Jonah there) later says it all happened in 3 days.
In the time of judgment we should consider with God one day is as a thousand years but a thousand years is as a day. Some may be in Gehenna a long, long time. Others may be destroyed in a day.