I get scared when I think, "What if there's someone behind me right now?"
I get paranoid when I think about someone being there behind me.
When I think about someone being behind me in the shower, I get freaked out and shut my eyes and can't wash my hair anymore.
１番目と２番目は「背後に人がいたらって考えると怖い」の翻訳で、３番目は長い方の翻訳です。「目をつぶる」は英語で「close / shut your eyes」と言います。
あとは、「超怖い」は直訳的に「really / super scared」と言えますが、「freaked out」や「paranoid」も強くていい表現です。
I think I'd get really freaked out if someone were standing behind me in the shower, so I don't close my eyes when I wash my hair.
この場合、次のような言い方ができますよ。
ーI think I'd get really freaked out if someone were standing behind me in the shower, so I don't close my eyes when I wash my hair.
「もし背後に誰か立っていたらすごいビビると思うから、目をつぶって髪を洗わない」
to get freaked out で「ビビる」
someone were standing behind me で「誰かが私の背後に立っていた」
ご参考まで！
アメリカ合衆国 
カナダ 
