How can I stop worrying about where I will spend eternity?
You stop worrying by placing your faith in the Gospel and resting by faith on the finished work of the cross, not on doing enough good to merit salvation. The free gift brings justification from many offenses; all sins have been canceled and paid for, so the believer need not wonder where eternity will be spent.
Les explains that Adam's one sin brought condemnation, but the free gift is of many offenses unto justification. This is beyond comprehension and must be taken by faith. He says he does not pretend to understand it, but he knows the Book says it, believes it, and can rest in it. Therefore he does not have to toss in bed at night wondering. He notes that even people who have preached the Word for a lifetime may start worrying as they face death, asking if they have done enough good. But none of us can do enough to merit it. The believer rests by faith on this finished work and should not worry about where they will spend eternity if they have placed their faith in the Gospel. The free gift is what we do not work for; though all are guilty of a trainload of sin, it has all been canceled and paid for by the work of the cross.