It's a peculiar insecurity. One easily solved by voracious reading. Like the gap left by a missing wisdom tooth, I will dwell on it until I forget to, then rediscover it again with the next unread tome.
I'm on a train north again; returning from a fleeting London visit to see my parents and catch up with some friends. I never have enough time in London. I always leave with a drop or two of regret, mulling those things I didn't do and those folk I didn't have time to see. The feeling lingers until after Newcastle, usually dispelled by the Northumbrian coastline and the calm that comes with crossing the border back into Scotland.
There's a young couple at my table on the train, restricting my nesting. My giant tome stays shut and I'm currently debating a nap. It's the flat part of the trip - there's not much to see outside. The sun's finally creeping out from the neutral overcast above. I'm struggling to stay awake, but I always worry I'll snore when there are people at my table.