All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
We ask Allah to bless you with recovery and well-being. You should know that it is impermissible to delay Ghusl after verifying that your menses have ended. This means that you are surrendering to Waswaas (devilish whisperings/obsessive doubts) and are getting carried away with it. What you should do is resist this Waswaas and not let it overpower you.
As soon as your menses end, perform Ghusl immediately. This does not take more than a few minutes. It is enough that you stand under the shower until the water reaches all parts of your body. It is also enough that you believe that water has most likely reached your entire body, and you do not need to be absolutely certain that it did. When you have done that, immediately get out of the shower and perform your prayer, no matter how much the devil whispers to you that your Ghusl was invalid. You should know that there is no remedy for Waswaas except ignoring it and paying no attention to it whatsoever.
Allah knows best.