Quote:

... how can it change so fast? Does the doubt ever go away? Will I every let my guard down and be completely happy?


Yes the doubts are quite natural and many before have ask the same questions on this forum (myself included ... Get out of jail FREE! ... page 4 ).
You may want to visit Matilda's thread as well ... Starting over -- one more time!

These threads can provide some insights to help with the doubts.