This is too good to be true.
→ 너무 좋아서 믿기지가 않아.

[예문]

A: Rachel, did you hear that Kevin and I are in the same class
    as you this year?
A: Rachel, 나하고 Kevin하고 너하고 같은 반 된 거 알아?

B: Yes, I did. I'm so happy.
B: 응, 알아. 너무 기쁘다.

A: Can you believe that all three of us are in the same class?
A: 우리 셋이 모두 같은 반이 된 게 믿어지니?

B: I know. This is too good to be true.
B: 맞아. 너무 좋아서 믿기지가 않아.