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 'alayhi wa sallam, is His slave and Messenger.
You should know that the religion of Allah is easy; all praise be to Him. A proper Ghusl (ritual bath) may be performed in much less time than what you mentioned. The Prophet, sallallaahu ‘alayhi wa sallam, was keener on performing proper ritual purification than anyone of us, and he used to perform Ghusl with one Saa' of water, which is approximately two liters.
As for what you are doing, it is all caused by Waswasah (satanic whisperings) and it is as far as it could be from the spirit of Islam. Allah, The Exalted, spared us hardship and difficulty. Therefore, you should discard such whisperings and know that it is adequate to believe that water has most likely reached the whole body during Ghusl; absolute certainty is not required in this matter.
Hence, you should perform ablution in the beginning of Ghusl after washing your private parts. Then, you should wash your head until you believe that water has most likely reached the roots of the hair (i.e. the scalp) and then pour water over the right side of your body and then the left, without exaggeration or overburdening yourself with undue hardship. This was the manner in which the Prophet performed Ghusl. If you stood under the shower for a while until you believe that water has most likely reached your entire body, this would be sufficient to remove the state of ritual impurity. The matter is easy, but you are making it difficult for yourself.
You should disregard such whisperings and pay no attention to them; taking them into consideration and acting upon them incurs grave evils on you. The way to treat such whisperings is to disregard them. Whenever Satan whispers to you that you did not perform Ghusl properly or that water has not reached a specific part of the body, simply disregard such thoughts and do not repeat Ghusl on account of these whisperings.
We ask Allah to bless you with wellness.
Allah knows best.