Travel solo often and never miss formal night - it's what cruising is supposed to be! Elegantly dressed people, wonderful food and drink, great music - of course, would enjoy it more with a handsome man in a tuxedo escorting me, but, in the meantime, I feel fortunate to be able to be in such luxury.
You mentioned Carnival. If that is the cruise line you are on, or NCL, then you really don't need to go dressed to the nines at all. They don't even call it formal night anymore. Not that jeans and tshirt would be okay, because that would not be appropriate, but anything nice is fine. I usually take a pair of black pants, some solid color tank tops and 2 semi sheer over blouses. Add a pair of sparkly earrings and I'm good to go.
Not going to comment on the high end cruise lines because I know nothing about them!
I wear a coat and tie everyday so am not interested in wearing em on vacation. I have a solo cruise coming up and will bring a blazer just for dinner in one of the specialty restaurants. On formal night, I may try room service then head out for a few drinks and enjoy the evening.
We also had to dress for years everyday at work an occasions. Now that we're retired and on vacation it's time to relax knowing that dressing up isn't our thing any longer and we just dress to please ourselves, not the others on the cruise.