You may be using muscles you have not used in the past for these tasks.
Initially it can be “expensive” to maintain better positioning because you are unlearning a habit while learning a new position/habit.  You are creating new neuro pathways for the movement/position. As you work with maintaining the changes the neuro pathways are grooving. As this continues it will just become the way you stand and you won’t need to think about it so much.
As with breaking any habit it is hard and it takes time.

Some episodes that hit the areas you mentioned:
Jill Miller Smashes Your Guts! (and psoas, and tacked down viscera, and matted down abdominals…) Part 2
Episode 301: Psoas Flossing and Biker Hips
Episode 129: The Twin Bowstrings, the Psoas and Scalenes
Episode 75: Your Inside Psoas Bits, World Record?
Episode 40: USAW Oly Prep: Some New Hip Love /Psoas/Calves
Episode 04: Silent P in Filet, Very Paleo

Episode 142: Tight IT Band and Hip Flexor Fix: Runners?
Episdoe 23: Runner’s Legs