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.