"It was or they were?" in this case

Solution 1:

The dummy subject "it" introduces an extension of the topic in your sentence. The real import of your statement goes beyond the mere fact that there were stories to explain what those stories meant for your understanding of your grandparent of his life. So your instinct to use "it was" instead of "they were" was a good one. You don't need number agreement with the previous clause.

(Note that you should keep the verb tense the same: "My grandparent told me ... such stories helped me ..."; and there were multiple stories, so I'd pluralize difficulty: "the difficulties he used to face"; also, why "grandparent"? You identify him as male, so why not grandfather?)