sb's heart isn't in it

If your heart isn't in it, you do not feel interested or enthusiastic about something.

心思不在…上面，对…不感兴趣

I tried to look interested, but my heart wasn't in it.

我试图装出感兴趣的样子，可是我的心思完全不在这上面。

Allen was studying to be a doctor, but his heart wasn't in it so he decided to follow his real dream and study film-making instead.

艾伦之前一直在学习成为一名医生，但他的心思并不在这上面，所以他决定追随自己真正的梦想，转而学习电影制作。