There was someone I used to work with who, well aware
of their tendency to look at the negative side of things,
used to constantly tell themselves to "think more
positively". 

This was said with such a feeling of desperation that I just
knew that not only would they NOT think positively, they
would probably go even further into negative thinking.  The
mere fact that they kept pushing themselves to "think
positively" was a sign that this would ultimately be
counterproductive.

So many of us have interpreted the desire to be positive
thinkers in a very narrow and ultimately ineffective way. 
The push to be positive often results in ignoring our
true feelings.  Instead of recognizing that we don't always
feel positive and learning how to convert and turn round our
feelings, it so often ends up with us denying them
altogether.

The Law of Attraction, as defined by Abraham Hicks states
that "you attract to yourself what you give your attention
and energy to, whether wanted or unwanted".

One reason why positive thinking doesn't work is that our
focus is in the wrong place.  So we say "I don't want to be
ill" and believe that this thought will prevent us from
being ill.   So where is our focus?  On being ill!
Because the Universe doesn't recognize "not" or "don't". 
How many times have you told yourself NOT to forget
something and then what happens?  Oops you've forgotten it!

Another reason why positive thinking doesn't work is that we
often veil our negative feelings with a positive surface.
positivity.  So we say "I want a loving relationship". 
Sounds good right?  Underneath this is the hidden message
that having a loving relationship wouldn't even be an issue
- unless you'd been in one or a series of not so loving
relationships. 

So it's the hidden message and negative feeling around it,
that is still coming through.  Ultimately, you're still
pushing against something and it's still
counterproductive.  You're likely to attract more unloving
relationships, even though you THINK you've stated
otherwise.

How can you begin to reverse this?  Rather than focusing on
your thoughts, focus on your feelings about a given
situation or what you would like the situation to be.  Start
to visualise and sense what you truly desire.  Focus on
ramping up good, positive, energetic feelings about what
you desire, rather than the thought. 

Practically you can do this by building a "creation box" of
pictures, ideas, notes, to stimulate your imagination of how
you'd like things to be.  Appreciating lifes abundance in
what you already have by keeping an appreciation journal,
feeling and acting like the prosperous person you actually
are and channelling your good feelings into what you desire.

In fact, instead of being a positive thinker, aim to become
a positive feeler - it's much more visceral and real.  Live
your life as though you already have what you desire and
you'll make way for it to come to you.