Your premise is incorrect. The assurance of salvation exists following baptism, and/or following sacramental absolution of sin, till a mortal sin is committed.
My premise is that in Catholicism "assurance" does not mean "to be sure". Instead, and notwithstanding your explanation that avoidance of mortal sin is achievable (but how many succeed?), it really refers to a temporary or fleeting state. One can be absolutely sure in a particular instant, but have no assurance concerning 5 minutes from now. To me, that is no assurance at all. There is no sense of certitude that would normally be associated with a word like "assurance". In Catholicism, it seems more like an uncertain "hope".
I'm having a similar experience in my discussion with Joe about the meaning of "eternal life". He has explained to me that having "eternal life" does not mean that we will have life with Christ eternally. Instead, it means that we will have Christ with us as long as we abide in Him, but that we can lose "eternal life" at any moment, based on our actions. Again, there is no certitude about the word "eternal", it is only a "maybe". Just as above, "eternal life" seems to be more of an uncertain "hope".
We have been arguing about the ABSOLUTE ASSURANCE of eternal salvation. We Catholics believe that we can have moral certitude of salvation, but never absolute assurance. In other words, say a 99% chance? I apologize if my defense of the difference has caused you to think we are not sure of our salvation. We are sure - to the degree that tommorrow we will not be attacked by terrorists with a nuclear bomb. If you read the lives of the saints and Church Fathers, they explain that we can never merit the grace of final perseverance - from our point of view - because we really don't know God's plan for us in the future. But after all of our arguments are said and done, FK, we have "assurance" that we will go to heaven. As to mortal sin, a Catholic abiding in Christ will very rarely do such a thing, so there is no "fleeting" back and forth as you propose.
It is hard to explain the mindset. WE try to remain humble and realize that God can do what He wills - even send us to the hell we so deserve. We do not want to sound presumptuous as if God owes us anything. On the other hand, we fully trust that Christ will not abandon us. All there is left to do is persevere - and we shall have Eternal life in heaven.
Regards
I know that it seems untherapeutic. However, it is entirely up to your free will to sin or not to sin. Don't sin, and bingo, absolute assurance. As St. Francis said, sainthood is easy: all you need to do is want it.