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very difficult to ...; nearly impossible to ...
「難い」 has other readings with different meanings.
この話は信じがたい。
This story is hard to believe.
彼の言い訳は理解しがたいが、聞いてみた。
His excuse is hard to understand, but I listened.
忘れがたい思い出になった。
It became an unforgettable memory.