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。