Some thoughts on this.

I'm really glad you changed your mind about the ending. I always found it brilliant, because the fact that Frodo fails is a key part of Tolkien's message about absolute power. It will always corrupt - slowly if your intentions are good (as Frodo's are) - but in the end, if you hold onto it too long, it will overcome you. The whole of human history proves this.

Also, there is the spiritual dimension to Tolkien's work. I know he didn't consciously design The Lord of the Rings as a Christian allegory, but his faith is bound to have informed his work, and the idea of the Ring/Sauron as tempter is powerful and profound. There's a clever ambiguity at play throughout the novel, as whilst the Ring is altogether evil, it doesn't excuse the bearer either. Their own inner weakness and evil is awakened by the Ring, and that manifests in different negative ways.

Our heroes go as far as they possibly can, but, as Gandalf says, "the pity of Bilbo may rule the fate of many". Had Bilbo killed Gollum, he wouldn't have been present at the end to attack Frodo and thus create what is (in my opinion) one of the finest examples of "crisis within the climax" in literary history.

There's even a sense of divine intervention, perhaps. Of a higher power who knows Frodo did everything he could, but that ultimately, a merciful bit of assistance was required. Again, I suspect Tolkien's faith is behind this feeling inherent throughout the whole novel. Gandalf refers to other powers besides the will of evil that meant Frodo to have the Ring. Also, there's that lovely moment when Sam sees the star far above Middle Earth, and recognises there are powers over which Sauron has no dominion.

What films did I change my mind on? Although I liked it to a point, I didn't consider Vertigo one of Hitchcock's best when I was a teeanger. However, every viewing down the decades made the film better in my eyes, to the point that I now consider it not only Hitchcock's greatest films, but one of the five greatest films ever made.

Also many of Ingmar Bergman's films failed to strike a chord with me as a teenager. As filmmaking I was aware they were great, but I didn't emotionally resonate with them until much later, as an adult.