「〜すると〜できない」と言うには、If I do X, I can/can't do Y. もしくは I can/can't do Y if I do X. のどちらでも言えます。日本語だと一通りですが、英語になると節を入れ替えても同じ意味になります。
英訳1：「枕を変えると眠れないんです」
英訳2：「違う枕を使うと眠れないんです」
If I use another pillow besides this one I struggle to sleep at night.
Whenever I sleep on a different pillow, I find it hard to sleep.
*If I use another pillow besides this one I struggle to sleep at night.
To struggle means to have difficulty in doing something. This means that you have a favorite pillow that is comfortable to sleep on and you are so used to it that you can only use that one only.
*Whenever I sleep on a different pillow, I find it hard to sleep.
This means that every time you use a different pillow, you struggle to sleep.
*If I use another pillow besides this one I struggle to sleep at night.
To struggleとは、何かをするのが難しいという意味です。快適に眠れるお気に入りの枕があり、それに慣れているのでそれしか使えないということを説明しています。
*Whenever I sleep on a different pillow, I find it hard to sleep.
違う枕で寝ると、いつも眠りにくくなるという意味です。
I can't sleep with just any old pillow. - this means that you are very particular and picky about the pillow that you sleep with
any old - this doesn't mean that the pillow is old,just that it is commonplace, ordinary, not special like your pillow.
e.g. She isn't just any old girl-- she's my girl!
A. You seem tired. What happened to you?
B. I got a new pillow because my old one is tattered.
A. Well, how is that such a bad thing?
B. It's bad! I can't sleep with any other pillow, so I couldn't get any sleep with this new pillow.
I can't sleep with just any old pillow.
- 眠るときに使う枕には強いこだわりがあるという意味です。
any old - これは枕が古いという意味ではなく、自分の特別な枕ではなく、ありふれた普通の枕という意味です。
【例文】
She isn't just any old girl-- she's my girl!
（彼女はそこらへんにいる女の子じゃないんだ！僕の彼女だから！）
【例文】
A. You seem tired. What happened to you?
（疲れてそうだね。どうしたの？）
B. I got a new pillow because my old one is tattered.
（古い枕がぼろぼろだから新しい枕を買ったんだ）
A. Well, how is that such a bad thing?
（そうか、それがそんなに悪いことなの？）
B. It's bad! I can't sleep with any other pillow, so I couldn't get any sleep with this new pillow.
（悪いんだよ！違う枕では眠れないから、この新しい枕でも眠れなかったんだ）
You have one very comfortable pillow - perhaps it is made with the very softest duck down filling? You always sleep soundly and are never disturbed when you use your 'special' pillow! You previously tried other pillows but nothing compares to your special pillow!
"No other pillow can substitute my special pillow!"
とても快適な枕をお持ちなんですね。
もしかして柔らかなダックの羽が入っていますか？
あなたにとっての「特別な枕」を使うといつでも熟睡でき、眠れないなんてことはありません。
以前他の枕を試したことがありましたが、特別な枕と比べものになりませんでした。
No other pillow can substitute my special pillow!
私の特別な枕に変わるものは他にありません。
If I am to get to sleep, I really need my usual pillow!
I cannot sleep on a STRANGE pillow...I need my own!
We all get into habits and we like the familiar...
When it comes to sleeping in strange surroundings it can be difficult.
It is perfectly normal to want to feel secure at night ......and your own pillow can make all the difference...So best to take it along with you if you really need it!!
To change something means that it is different from what you had before so as you could say
'I can't sleep with a different pillow' or 'I can't sleep if I change my pillow' both meaning the same thing
To find something hard means it is not easy to do and you struggle doing it so you could say 'I find it hard to sleep with a different pillow'
To change something'は「～を変える」という意味です。ですから、以下のように言えます：
'I can't sleep with a different pillow'（枕が変わると寝られない）
'I can't sleep if I change my pillow'（枕が変わると寝られない）
どちらも同じ意味です。
'To find something hard'は「～を難しいと感じる」という意味です。ですから、以下のように言えます：
'I find it hard to sleep with a different pillow'（枕が変わるとなかなか寝られない）
When you want to explain that you cannot sleep with a different pillow; then you can say it in the following ways:
-I don't like changing pillows
-I can't sleep with a different pillow
枕が変わると眠れないなら、以下のように言えます：
【例文】
-I don't like changing pillows
［訳］枕が変わるのは好きじゃありません
-I can't sleep with a different pillow
［訳］違う枕では眠れません
アメリカ合衆国 
日本 
南アフリカ 
イギリス 
