I beg to differ - the worst part of a cruise is leaving the ship on the day your cruise is over!
If you are really that anxious about it, ask your doctor if he/she can prescribe something (mild!) for you to take to ward off those fears. And carry along a little Bonine or non-drowsy-formula Dramamine (make sure it will be compatible with any prescriptions you are taking though!!) in case you do encounter a bit of motion discomfort. Chances are, within a day or two you will be fine. And you can try the "Peter Pan" treatment - think happy thoughts (in this case, focus on things onboard you can enjoy - there are lots of activities on sea days) and try not to focus on your fears.
My first cruise, I made sure I took some Dramamine (didn't know about the non-drowsy kind at the time) before we left, as I was very afraid of getting motion sickness. NOT a problem - after the first day, I didn't need it at all.