If we don’t repent of our sins the atonement has no effect on us. We have to suffer for our own sins. Is this correct?
This is correct according to the following scripture,
15 Therefore I command you to repent—repent, lest I smite you by the rod of my mouth, and by my wrath, and by my anger, and your sufferings be sore—how sore you know not, how exquisite you know not, yea, how hard to bear you know not.
16 For behold, I, God, have suffered these things for all, that they might not suffer if they would repent;
17 But if they would not repent they must suffer even as I;
18 Which suffering caused myself, even God, the greatest of all, to tremble because of pain, and to bleed at every pore, and to suffer both body and spirit—and would that I might not drink the bitter cup, and shrink—
19 Nevertheless, glory be to the Father, and I partook and finished my preparations unto the children of men.
20 Wherefore, I command you again to repent, lest I humble you with my almighty power; and that you confess your sins, lest you suffer these punishments of which I have spoken, of which in the smallest, yea, even in the least degree you have tasted at the time I withdrew my Spirit.
The eternal principle of justice demands that all debts be paid. Sin is equal to incurring a debt. The eternal principle of mercy allows another to pay the debt if they are able. Christ has offered to pay all debt, but we must accept the terms of his offer. If we do not then we are left to pay the debt ourselves.
In other words, mercy is laying claim on our souls through the Atonement of Christ. However mercy cannot rob justice. If we are not obedient to the merciful terms of Christ, then justice has full claim on us.
The beauty of the plan is that the terms of the atonement are easy and simple compared to the terms of justice. Christ has paid the price and all we must do is follow him in righteousness as much as we can.