Non-derogatory term similar to "inflexible"

Solution 1:

While "inflexible" may be a component the traits you describe, you are not really looking for a synonym of "inflexible". The person you describe can be said to be disciplined or regimented.

Both words connote a regular and strict conformity to a course of behavior. They are often used to illustrate that the behavior is for the person's overall benefit. This might be the case even while there may be some negative consequences, the least of which may simply be the lack of flexibility that their regimen allows.

Solution 2:

A neutral word that comes to mind is adamant.