How can I not feel guilty after my mom developed a bed sore with my caregiving?

Learn. My best advice is to engage the help of a rehab nurse. They can teach you the best ways to decrease the pressure on vulnerable areas of the body which are typically the back of the head, the shoulder blades, the sacral area and the heals. These are all the parts that are closest to the bone. Learn about turning schedules and special beds that decrease pressure sores. Learn how to prevent them.
It can be. Difficult. Don't beat yourself up. Even christopher reeve developed ulcers and he had lots of support. Making a schedule is best and having a team approach.
Pressure Ulcer Blame. There are many factors that cause pressure ulcers. Even with the most intense care, pressure ulcers can be unavoidable. I refer to to google "kennedy terminal ulcer" which as noted developed in the mother of ms. Kennedy with two caregivers working full-time to prevent injury. Wound care professionals recognize that many factors affect the impact of any form of pressure on the patient.