It is discipline, oddly enough, that allows one to change - But when one is trying to change into something more care-free, the disciplinarian is the one that will have the most trouble...Go figger.
I am having a similar journey... All the years in a wheelchair left me with a fairly sedentary mindset, unbeknownst to me. The work and weight-loss regimen have not been fruitful, and the reason why is that sedentary mindset.
To wit: Active people eat whatever they want, don't necessarily follow exercise regimens, and are never overweight... for no other reason than their being active... The physical result is not regimented, and is tangential to the point...It is the being active that naturally produces the outcome... Hence, rather than worrying about the weight, worry about the mindset, eh?
Becoming exuberant is somewhat the same sort of thing... Although 'happy' is not what I am shooting for- Happy to me, is ethereal, capricious... unable to be maintained... It is a reaction to a set of circumstances, and must be enjoyed in the moment... The important part there is to actually experience that moment to the fullest when you find it. Allow yourself to be happy.
Contentment...now that is the thing. One can optimize contentment in nearly any circumstance. I cannot be happy about being broke down on the side of the road - but while I am waiting, a dry comfortable spot, and afire, can bring contentment, even in the midst of the trial.