All my life, no matter how good I feel about getting into a good
groove (working out and eating right) I always "backslide"
and stop doing well.

What's the latest thinking on why people suddenly stop 
positive trends in their life like this, and go back to bad habits?

The answer I'd like to offer is simply, they are bad habits. It is all
psychology, motivation and some psychosomatic input. People get back
into eating badly and stop exercising because of addiction to old
habits and in some cases the body physiologically can't take a change
that's rapid and forced. Weaning into exercise and diet is helpful,
take it slow and find new ways to motivate yourself. Have realistic
goals, not just "cold turkey" have junk food every now and then, just
take it easy, and make sure you find an exercise you enjoy,
walking/football/gym/jogging/sel defence.

Many psychologists have done studies on this behaviour and I found a
very interesting book on the subject! You have to buy it, but it is
worth the read and delves deeply into the mind behind the habit!

Psychology of Exercise
Author(s): Lox; Martin; Petruzzello

Can be bought on: http://www.ecampus.com/bk_detail.asp?isbn=1890871478&referrer=pdinb

You have to keep on making the correct choice.  Will power and
motivation can be low on a given day.  Good results generate a
feedback loop of encouragement.  That can either be positive (you look
great) or negative (my father had a heart attack at age 40). 
Depending on the strength of the motivation and the urgency you then
decide to sleep in our go to the gym.  Also, I've noticed that
motivation levels change after an event or when a goal is reached. 
The wedding is over, the reunion or vacation has come and gone, the
weight was lost, the race was run, the time goal was reached.  Now
what?  New goals or a new motivation or reason for continuing is
needed.  Friendships at the gym might replace the weight loss goal.  I
think another factor is accountability.  If I miss TODAY will anyone
notice or care - which of course has to be answered every day.  Having
someone who is going to meet you is critical to long term success.  I
find urgency and accountability help in flossing.  When I have an
Dentist appointment coming up my commitment is much better.

Its about good and bad omen. but if u persist your aim and dont be
lured away by bad one, ultimately truth or the right prevails.in India
it is called "satyamev jayate",in sanskrit language.just like ur
stamina level u have to develop persistance power, in other words
"stick-to-itiveness".