Adjective
reticent - Describing someone who is inclined to keep their thoughts, feelings, or personal matters to themselves, not expressing them freely.

He was reticent about discussing his past, thinking that there were details that she might not react well too.
regimented - Referring to something that is arranged in a highly structured or ordered manner, often with little room for flexibility or deviation.

He was determined to lose weight, and he realized that he could accomplish this only with a regimented approach to exercize and diet.