Hard to believe with 〜がたい
Grammar Explanation: Difficult to do with 〜がたい

A student from a lower grade enters the classroom. She stops and stares at Asuka Jr., one of the boys in the class.
- 涼子:
- 「ある特定
( のところに集中( してない? しかも」- “Isn't she focused on one particular area? And on top of that...”
- 恭子
( : - 「ものすごく信
( じがたいところに……」- “It's in a frightfully hard to believe area...”
Key Points
信じがたい means hard to believe
〜がたい attaches to a verb stem, so 信
( じる becomes 信( じがたい.
ものすごく intensifies the difficulty
ものすごく means extremely or terribly.