I cannot find the urethral opening. Can you give me some advice?

The opening can indeed be so small that it is hard to find her. Sometimes she is close to the clitoris, but she can also be all the way down near the vaginal entrance. The easiest way to find her is to take a small mirror with you when you are going to take a shower. Kneel, open the inner lips and pee. Use the mirror to see where it comes from.