It is very difficult not being able to tell people what I am thinking.
It is very frustrating not being able to tell people what I am thinking.
It is very frustrating not being able to express what I am thinking.
こんにちは Kothyさん、
1) It is very difficult not being able to tell people what I am thinking.
What I am thinking = 自分の思っていること
Not being able to tell = 伝えられない
It is very difficult = とてもつらい (この difficultは感情的に難しいの意味です)
2)It is very frustrating not being able to tell people what I am thinking.
It is very frustrating = とてもつらい
3)It is very frustrating not being able to express what I am thinking.
Not being able to express = 伝えられない
ご参考になれば幸いです。
It's so frustrating when you're not able to tell people what you're thinking.
「自分の思っていることを人に伝えられないのがとてもつらい」は以前のジョニーさんの回答で大丈夫ですが、もう一つの言い方として”It's so frustrating when you're not able to tell people what you're thinking. ”があります。
そして、「英語でコミュニケーション取れないのがつらい」は”It's so frustrating not being able to communicate in English"で大丈夫です。
ご参考になれば幸いです。