What you described happened in complete ignorance and at a very young age. In Islam, accountability is tied to knowledge and awareness. A person is not sinful for something they genuinely did not know.
About fasting If your menstruation had actually ended before Fajr, but you did not perform ghusl because you didn’t know it was required — your fasts were still valid. Ghusl is required for prayer, not for the validity of fasting. Being in a state of major impurity does not invalidate a fast; active menstruation does. So if the bleeding had stopped, your fasts are not invalid.
About Praying without ghusl after menses is considered invalid. However, here the issue is ignorance. If you truly did not know that ghusl was required, and this was long ago with no clear memory of how many prayers were affected, then Islam does not require you to start guessing and reconstructing the past.
There is this principle that a person is responsible once knowledge reaches them. Islam does not demand that you calculate vague numbers based on uncertain memories. If you feel uneasy, you can increase voluntary prayers or voluntary fasts for your own peace of mind — but you are not required to randomly make up unknown amounts. Religion is built on clarity, not retroactive anxiety. What matters now is that you know. From now on, you act correctly. The past done in genuine ignorance does not trap you.