Not quit, but cut out, snip away at, remove bit by bit.
Two things to focus on changing:
1) A mental habit or worry that is eating too much of your energy or time.
2) A project or routine that isn't going anywhere or which distracts you from things you really wish you'd do more on.
– When you start to fret about whether you can get a date or what went wrong with a past one, use this as your reminder to do something else. "Oh, I that's right, I don't get to spend 30 minutes worrying about whether Pat likes me until after I do my stretching routine." Do that other thing and then – if you still remember – decide if spending the next 30 minutes on fretting is really what you want to have lined up next in your day.
Force spending time fretting to be a conscious decision about how to spend time; it will almost always lose out to other options since it makes you feel so silly to be choosing it.
– Want to stop spending hours watching TV and finding the evening gone? First, make sure you watch on your own schedule. Use a PVR like Tivo or even an old VCR to record shows so you aren't chained to the broadcasters' clocks like that guy in Metropolis. Second, always "buy" your TV watching time with equivalent time spent on one of your top two projects. I recommend having a creative thinking project such as writing and one that's more physical which doesn't require much mental energy so that you can make progress in different moods.
Over time, swap time that doesn't reward you for time that does.