So what now? I am going to hell for having sincerely believed and, over the course of several years, come to disbelieve? This is a ridiculous. I do not see how it is a tenable position in light of various other positions that the Church claims to hold, e.g. non-Catholics may be “saved”, Orthodox Christians have valid sacraments, etc.
I do not believe that anyone could say whether you’re going to hell or not. The Catechism makes exception for those who do not know the Truth through no fault of their own. Does sincerely coming to disbelieve, such that the person just could not even will themselves to believe in Catholicism if they wanted to, count as being no fault of one’s own?

I honestly don’t know.
I think that most people are concerned with the possibility that anyone who has been Catholic before may well have, at some point, had everything they needed in order to be Catholic for the rest of their lives, and at some point might have made a faulty/immoral choice that
enabled them to reach the point to where they could no longer believe. In that case, the line is blurry: True, the person
now cannot believe, but can we say it was through no fault of their own if the initial means that set them on this path was due to some moral misdirection they inflicted upon themselves earlier? Will God decide whether they are “at fault” based on now and the future, or will He consider that they certainly had their chance to keep believing and, at some point (perhaps forgotten by them now, as unsettling a possibility as that is) did something that weakened that, thus they themselves are at fault for their unbelief? Again, I honestly don’t know.
So you see, there are too many variables for me to be totally at ease with someone being non-Catholic. I can honestly say I do not know they are definitely going to Hell, but nor do I have enough knowledge (nor am I sure they themselves can) to know confidently that, in the Catholic context, they will definitely going to Heaven. I just don’t know. My uncertainties are why the only thing I can say for certain is that it is safest, by far, to be a devout, practicing, believing Catholic,
from my Catholic perspective. I do not know what will become of a given non-Catholic or a Catholic who leaves the Church, but I do know that a devout and practicing Catholic goes to Heaven. When faced with one path that is less than certain, and one that certainly leads to Heaven, who can fault us for cautioning people against following the path we have found to be the uncertain one?
Again, this isn’t to start a debate on whether Catholicism is true (that’s not the point of this thread) but just to show why it is not uncharitable or “condemning” for someone who does believe in Catholicism to caution someone against leaving Catholicism or staying outside of the Church. We only know for sure what happens to a devout Catholic, which is eternal bliss with God, so of course we would caution anyone to take that safer route. As for your own opinion as to what is true and what will happen to you, you must work that out with God yourself, and I will pray for you.
Blessings in Christ,
KindredSoul