Hi,
Hope you're hanging in there ok.
I can say for myself that starting a thread and opening up to people, even anonymously, is not easy, so I tip my hat to you for doing so.
You piqued my interest with that gemara in sotah, so I took a peek at it.
Before going into specifics, a wise Rebbe once told me that all the authors of the ma'amarei chazal which speak of punishment etc want to inspire people to not sin. If learning them causes someone to feel down and unmotivated, the tannaim and ammoraim who wrote them would be saddened by the effect it has had on that person! It means they are doing the opposite of what was intended!
I think its fair to say that the all of these scary sources want us to do teshuva, and when I keep it in mind when I come across sources that make me feel unmotivated, it helps me.
It seems that the gemara in sotah doesn't mean that all the mitzvos someone did counts for nothing, rather that one aveira can counteract one mitzva. The word mitzva used there is in the singular, so it doesn't undo everything.
Also rashi says that the aveira undoes the schar, or reward of a mitzva. What is Rashi adding? It's undoing the schar of the mitzva as opposed to what?
I think it's possible to answer that other than the reward one gets for a mitzva there is the value of the mitzva itself, and how it brought the person closer to Hashem. So when Rashi says that the reward for that one mitzva is taken away, it doesn't mean that the mitzva counts for nothing. Hashem still cherishes it and sees the effort and brings the doer of that mitzvah closer to Him.
Hashem isn't out to get us, and He wants us to come closer to Him.
Please keep us updated on your progress!
(And yes, falling doesn't mean that no progress was made
All the best!
jj123