A few days have passed and unfortunately something "bad" happened. First, I made progress. Yesheon and I continued cultivating every night for about 30 to 40 minutes, which was her limit.
We even tried to stay longer, but after an hour, she started feeling unwell and we had to stop. Even though she said she was fine, it was obvious she wasn't. After studying a bit about cultivation, I saw how it can be harmful to the body.
So it's something that must be done with a lot of moderation and caution. That's why we went back to doing only 30 minutes, which was the recommended time between us without her getting sick.
And with that, I solidly achieved the formation of the core without any problems, becoming as strong as Lyuei. Although after that, my power didn't have that many increases, after all, it's just a few days of training.