My daughter has grown up since she's become a high school student.
These days, I'm beginning to not understand what she is thinking.
Maybe it's time for both of us to become more independent from each other.
ご質問ありがとうございます。
"My daughter has grown up"＝「私の娘は大きくなりました」
"since she's become a high school student."＝「高校生になってから。」
"These days, I'm beginning to not understand what she is thinking."＝「最近、彼女が何を考えているのか分からなくなってきています。」
"Maybe it's time for both of us to become more independent from each other."＝「もしかして、お互いから自立する時期が着ているのかもしれません。」
ご参考に。
アメリカ合衆国 
日本 
