It might be that your Grandad is more ashamed of his state of terror at the time being, than of the simple fact that whatever emotional state he was in at times, he still should feel pride at his war service and his due recognition, not the least from himself, that he was a hero.
Never having been in a war, I still think if I were in the midst of it, exceedingly shameful moments of, say, uncontrolled bowel movements, is not an uncommon phenomenon that I might be subject to.
Other indicators of personal "cowardice" might be equally unpleasant memories.
War is hell, I think, and perhaps you should respect your Grandad's reticence.
Not because he was not a hero (I'm sure he was), but because it forced upon him insights into what he regards as weaknesses in his own person, and therefore is unwilling to communicate to others.
Oh, I think you misunderstood what I meant. I never once pressed him to talk about it. I would never do that. And above all, I do think he deserves the respect. He is a great man and I would not pressure him into anything.