When does change occur?

Although i really enjoy people that have great inspirational messages, I've never put that much stock in these professional inspirational speakers.  You know the ones that come to a town, rent out a conference room, charge people $25 to encourage them to "UNLOCK YOUR POTENTIAL!"  In 3 months - if you buy their books - you'll learn tips and tricks that will make you thin, rich, famous and you'll have great sex every night and fart huge rainbows.

Yeah...

But today I ran across a short video by Tony Robbins.   It started playing and I was half-listening.  I wasn't sold on everything he talked about, but it was a positive non-judgmental message.  

And one thing he said really hit home.  He said that change does not occur when you think you "should" do something, it occurs when you "MUST" do something.  So saying to yourself "I really should lose weight" or "I should totally start saving money" means NOTHING.   Not until you  change it into a MUST.

For years I knew that I should lose weight.  But until something clicked - that I HAD TO change - well, nothing changed.  Then it became non-negociable.  A must.  And I lost weight.

So I wonder if I'm back to "should".  Not with working out - that is still a must.  And I do work out faithfully.  Because it is still in the has to happen category.

But food is a different matter.  That is, I think, back to "I really should get my eating back in check..."  Not "I MUST stop eating so much!" 

The question that Tony Robbins didn't answer is HOW to turn a "should" into a "must".  What is the key to the magic in flipping that switch?  How do I get back to putting my eating in the right column?  Is it reallly a simple mindset?