(one's) heart isn't in it
One is not emotionally invested in something. I used to love running, but my heart just isn't in it anymore—that's why I quit the track team this year. I can tell by the way you wrote this that your heart wasn't in it. Is something distracting you?
heart in it, have one's
Also, put one's heart in it. Be emotionally involved in something, undertake something enthusiastically, as in Nancy puts her heart in her teaching. This expression may also be put negatively as one's heart is not in it, as in She decided to quit; her heart just wasn't in this kind of work. [Late 1700s] Also see have no heart for.