Couldn't tell means you could not know or see.
Cry over somebody means that you are sad about him/her (probably because have lost him/her as a lover/friend). It is commonly used.
I thought I was over you means I thought I was no longer in love with you as you say. It is also commonly used.
For in this sentence means because.
From this moment on means from now on. (Note:- There is a song written by Cole Porter with the title "From this moment on", see below)
Your knowledge of colloquial English seems very good !!
Lyrics of From this Moment on +
"From this moment on
You for me dear
Only two for tea dear
From this moment on"
"From this happy day
No more blue songs
Only whoop dee doo songs
From this moment on..."etc...