...that I've been turning over in my mind ever since.为什么不是...that I've been turned over...

have been turned over 其实是have been done ,是被动语态,意思是我已经被xxx,have been turning over 是be turning over 的完成时,主语是我,意思是我已经xxx,两个意思完全不同

turn over 是你个人主观的反应，而be turned over 是被动形式，这显然是不对的。