Baseline posture is important! Everyone has a unique baseline posture. If someone has been sitting in a slouched posture for years, their body has adapted to that. These people will have a harder time getting their pelvis upright and that’s ok! It’s a journey and each individual should start at their comfort level with the goal to gradually improve their posture over time.
In depth answer: Anthros is an orthotic to improve your posture, like orthotics for your feet. There is an adjustment period as your body adapts to an improved posture, so you may need to take breaks during the first few days during the adjustment period.