# I have a baptism to be baptized with

Here "baptism" refers to what Jesus must suffer. Just as water covers a person during baptism, suffering will overwhelm Jesus. Alternate translation: "I must go through a baptism of terrible suffering" or "I must be overwhelmed by suffering as a person being baptized is covered by water" 

# how I am distressed until it is completed

"I am terribly distressed and will be distressed until I complete this baptism"