to this day I can't figure it out. It just seems like it's too hard to figure out.
It IS too hard to figure out, and especially with our own natural or unrenewed mind.
Isaiah 55:9
For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts.
This is why we need the Holy Spirit to speak truth directly into our spirits, which somehow, by a miracle, God enables us to understand spiritually, and as we receive that truth in our hearts, and mediate upon it, and grapple with it in our minds, He brings us through to mental understanding of something more than we 'knew' before.
But we need to give up all thought of subjecting His wisdom and knowledge to the scrutiny of our fallen minds. That, is a lost cause!
EDIT:
To be practical, you have to decide whether you are going to believe God's word. When that is settled, and your mind is fixed on getting to know Him through His word, you will find yourself gaining understanding. I say so with conviction, because of this testimony:
James 1:5 If any of you lacks wisdom, let him ask of God, who gives to all liberally and without reproach, and it will be given to him. 6 But let him ask in faith, with no doubting, for he who doubts is like a wave of the sea driven and tossed by the wind. 7 For let not that man suppose that he will receive anything from the Lord; 8 [he is] a double-minded man, unstable in all his ways.
With regard to your particular question, Paul Washer in his famous sermon The True Gospel, deals with the infinite nature and worthiness of Christ as our sacrifice. For the same reason, 3 days in 'hell' would be enough for all the people who will ever live, for whom He died. [It's here on SI, or, the video is on youtube.]
Regarding the concept of 'everlasting' or 'eternal', Young in his literal translation of the Bible, uses the phrase 'age-during', which thereby accommodates the major eras within which God has, or will, reveal Himself to mankind.
God has not revealed much about what will happen in the next age, but He reveals Christ to us, that He may be in us, that we may prove Him faithful to all of His present revelation, and thus honestly, consciously, deliberately, choose to please Him, that we may be with in in the next age, whatever that may hold for us.
Matthew 7:17 "Even so, every good tree bears good fruit, but a bad tree bears bad fruit. 18 "A good tree cannot bear bad fruit, nor [can] a bad tree bear good fruit. 19 "Every tree that does not bear good fruit is cut down and thrown into the fire. 20 "Therefore by their fruits you will know them. 21 " Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of My Father in heaven.
Anther thing to consider, is the perspective of the person dying. To aid you in this, read Ezeziel 18. But remember this was written a long time ago. A necessary question is, whether anything has changed because of Christ's sacrifice. Without doubt, to those who believe, it has changed, because we receive the power to become sons of God. But what of those who don't believe? Is their condemnation GREATER, since Christ has come?
It is useful to remember when reading the New Testament, that the word 'reward' doesn't always mean something good is going to happen to the person receiving it. God will treat them the way they treated Him and others, unless they repented if that was appropriate.