No one goes to sleep one day a broken person and wakes up the next day as the perfect specimen.
Anyone that tells you otherwise is lying not only to you, but to themselves.
You spent a lifetime building the person you are today, and that person likes it here, and will not be quick to phase out of existence. So when it comes to personal growth, you must offer them the time to adapt to a new set of rules.
When you are transitioning between two states of being, there is a phase where it becomes necessary to to compartmentalize who you are becoming, and who you were, but it is vital to remember this is not the final phase.
In order to fully adopt the new you, the old you must come to accept the terms of existence that the new you creates. Not as denial of their existence, but as acceptance and integration.
Take for example the child that wants nothing more than to be an adult. They can crave the maturity and freedom, and often times emulate it quite well some leave home and some get married early, but the part that they often overlook is that the growth that they sought out was not the acceptance of adults or the freedom of choice. They often have more of that than they think.
No, what they seek is a shortcut in their own evolution. They seek to forcefully eject the child from their identity. However the child in them, grown slowly over the course of their life steadfastly rejects it’s own death and clings on in self-preservation.
In order to effectively become the adult they wish to become that child must learn the rules and become comfortable with all aspects of the adult. This happens naturally over time when the old you is in alignment with the new you.
When attempting to change something against the nature of the old you, such as the case of the child who wants to grow up faster. Compartmentalizing is useful to allow the old you, to see the new you and adapt in a time frame that makes sense to them.
This is just one example and it applies to many different places in life where you are aware of of a change you want to make, you could apply the same thing to business, personal development, or quitting a habit like smoking and recovering drug addicts. Any place where a person does not want to end up where they were.
The important thing to remember is that new you offers the direction, the old you sets the pace of the transition.
Once the old you is comfortable enough with the new you that it is willing to sacrifice old traits, integration can begin and a new you is born.