My father struggles in R as well.
By my analysis, this is because he didn't carefully cultivate hobbies and outside interests during his working career. This had the effect of stunting his imagination, creativity, and drive to do anything outside of work. His identity is work. He can no longer see himself, say, picking up the banjo. That's artsy fartsy crap that only other people do. Or write. He feels this way despite wanting to originally wanting to be an English teacher and instead becoming a mechanical draftsman to support his family. "That ship has sailed," he said to me a year ago when I asked him if he was going to pursue his old dream of writing a novel. The interest is gone.
He had a prolonged period of life -- indeed, the majority of it, in terms of years -- which was defined by doing things he "had" to do. He had to go to work, perform his function, think about work at night, manage his family, take care of endless errands and chores, and so on. Weekends were booked with family activities because that is what was expected. His formerly vibrant inner world collapsed in on itself at some point during four and a half decades of employment, and his own 'wants' are basically gone -- or seem unattainable.
His major hobby is physical activity, which he's kept up, post-work. But that only takes an hour or two a day, and then he's left without much else to do. He can also watch sports, but there's only so much yelling at the TV that his wife (my stepmom) is willing to tolerate every week.
Lesson learned: Don't wait for retirement to engage in your hobbies. Draw, read, paint, play a sport, learn to cook, volunteer, camp out somewhere, build furniture, get more involved in your church or community, take classes -- whatever the heck it is that you feel you want to do in retirement, start doing it now, even if it's only on a limited basis, so that these things become part of your identity and it feels natural to expand the amount of time you're investing in these areas after work. Make sure you have something to retire to instead of merely something to retire from.
As others have already mentioned, I think if you are a creative, curious person and you cultivate these personality traits while working, you'll be fine in RE. Don't let work define you and then quit -- everything I've experienced and read seems to point to that particular path leading to a big fat happiness fail.