# if this cannot pass away unless I drink it

"if the only way this can pass away is if I drink it." Jesus speaks of the work that he must do as if it were a bitter liquid that God has commanded him to drink. 

# if this

Here "this" refers to the cup and the contents within it, a metaphor for suffering.

# your will be done

"may what you want happen" or "do what you want to do"