It is very difficult for me as a middle manager to keep a good relationship between my boss and the staff I manage.
「It is very difficult for a middle manager like me to keep a good relationship between my boss and my subordinate.」という文章でもオッケーです!(aを気をつけましょう!)「subordinate」は少し硬い言い方ですし、「軍隊っぽい」言い方なので「junior colleagues」や「the staff I manage」、「my team」、「the team I'm in charge of」などの言い方の方が優しいと思います。「subordinate」は英語圏の国では「ネガティブ」なニュアンスがあります。
下記のような英語になります。
It is very difficult for me as a middle manager to keep a good relationship between my boss and the staff I manage.
It's tough being caught between my boss and my subordinates.
中間管理職が上司と部下の間で大変な状況にある場合、"It's tough being caught between my boss and my subordinates." と表現します。ここで "tough" は「困難な」「大変な」を意味し、"caught between" で文字通り「間に挟まれた」という立場を示しています。
他には:
I'm in a tough spot as a middle manager, balancing the expectations of my boss and the needs of my subordinates.