I may have had the realization about the all-or-nothing idea when thinking about my meat consumption. I know that eating meat is very resource intensive. I also know that there's no way I'll make it through life without consuming a tasty burger or chicken every now and again (I think once a week would be the bare minimum I could tolerate). Not eating meat is better for the Earth than not driving your car. But you don't have to give up all the meat in your diet. You could just cut it back gradually. Every pound of pork passed by is a step in the right direction. But I love bacon. I also find (this may be surprising to those who don't know about diminishing marginal utility) that the less often I eat bacon, the tastier it is.
Let's say you have a problem with time spent on Facebook. You could delete your account. Or go for a week without it. Maybe a more sensible approach would be to limit your daily time on the site. Or maybe you think you drive too much. You don't have to trade in your Buick for a bicycle...just start making little trips on the bike instead of driving. I hope that you'll start to fall in love with cycling and gradually displace more car trips, but it'd be dumb to encourage anyone to go cold turkey on the car.
On the other side of the coin, say you want to start working out. You don't have to hit the gym 7 days a week. Maybe start a running routine at a reasonable rate. Don't try to pack in 50 miles/week right off the bat. You can gradually phase into good things and out of bad ones. Then you can stop phasing in/out when you find the balance that works well for you. The process will be more comfortable and each step in the right direction is a good step.