I'm aghast at how many important things I overlooked.
I can't believe the number of crucial occurences that I was clueless at.
「気づけなかった」はいろいろな英訳があります:
I overlooked it
I missed it
I failed to notice it
It passed me by
I was clueless at that
「愕然」は「astonished」「shocked」「amazed」でも言えますが、この場合には「aghast」が一番いいと思います。