We just did a B2B on the Dream at the beginning of April and yes, it is very tiring. We also were up to midnight most evenings and then up early the next morning for excursions, etc. What we ended up doing was seeing all the shows we wanted to see, character greatings, getting all the dress-up pictures taken, etc on the 4-night. Then on the following 3-night, we did our excursions during the day and took it easy at night. We actually only made it to the dining room on the first night of the 3-night cruise and just had room service or snacks the other two nights because we were burnt out, LOL.
It was very nice having that extra 3 nights on the ship to do some of the other activitines like riding the Aquaduck and playing Goofy Golf. If we had to leave the ship after the 4-night, I would have felt exactly the same way as you.
I am looking forward to our 7-night MR cruise next year because I expect that to be at a more relaxed pace (with 3 sea days).