Just had a smart conversation yesterday about this. It's been something I've been thinking on for a while.
There's a bit of a problem with long term habit change. If you're working on something that takes a while to achieve, you spend a lot of time falling short of your target and aware of it.
So, let's say you were currently drinking a lot of soda, and you want to quit.
You start replacing soda with other drinks, trying to order different things at restaurants, buy other things, turn friends and family down when they offer you a soda, get a bottled water instead of a coke at the movie theater with popcorn, etc, etc, etc.
Sometimes you go to a barbecue or a cheap lunch with pizza, and the only drink is soda. You try to just have nothing those times.
I'm reading "Seeking Wisdom: From Darwin to Munger" right now by Peter Bevelin, which is excellent. Author Bevelin is talking about fear here -
Fear warns us of potential harm and keeps us from acting in self-destructive ways. It helps us avoid threats and makes us act to prevent further damage. Fear guides us to avoid what didn't work in the past. Fear causes worry and anxiety, a normal response to physical danger. It activates hormones like adrenalin and cortisol, which keeps us attentive to harm since we need full attention to escape from a threat.
The degree of fear we feel depends on our interpretation of the threat and our perception of control. The more helpless and vulnerable we feel, the stronger our emotion for fear becomes.
He goes on to give an example of how your body will react if you're walking down a deserted street at night and hear noise behind you. You'll naturally start to breathe faster, grow more aware, etc. - an instinctive response. He continues:
What we fear and the strength of our reactions depends on our genes, life experiences, and the specific situation. You may react instinctively at first, but if the situation is one that you'be experienced before (since our brain is continuously being "rewired" with life experiences), the final reaction may be to calm down. [...] The more we are exposed to a stimulus, even a terrifying one, the higher our threshold of fear becomes.