How to take constructive criticism?

Neither of the examples you provided are somebody criticizing you, constructively nor otherwise. Another nurse, who was aware that it was your first solo shift, was trying to help you by teaching you the procedures and practices of your facility.

She's right, too; you don't want pharmacy associating you with any kind of narcotic error or discrepancy. You need to follow your facility's narcotics policies exactly. Perhaps a second signature was needed for beginning/end of shift counts, or for a dose that you documented as wasted, or perhaps the facility you work at requires two nurses to sign for narcotics each time they're pulled. If you're unsure of how things are done, especially with something like narcotics that can land you in hot water, you need to ask instead of guessing. It sounds like she meant it nicely.

Medications and supplies need to be in their designated spaces so that all staff members can find them quickly and without searching. This requires everyone to know where things go. If a patient is in respiratory distress, it's essential that the nurse is able to find the patient's inhaler quickly instead of searching for wherever the previous nurse may have set it.

Neither of these corrections was a criticism about you. Both were teaching moments. Whether you are a new nurse in a facility or you've worked there for 20 years, you've got to be open to learning new things and feedback from peers. Thank them for teaching you the important things you need to learn, and implement the correct practices. If that is a struggle, remind yourself it isn't about you; it's about patient care and patient safety. Everyone is working towards the same goal.

The only other thing I would add is ask questions - so many questions! If it was your first shift alone in this job, you don't even know what you don't know yet. When you across new procedures, new equipment, new patient situations - ask.