What is the ruling on my prayers and fasting, and what is obligatory for me to make up, after having practiced masturbation out of ignorance? And does the ghusl for menstruation or the ghusl for embracing Islam suffice for the ghusl of janabah?

The treatment for obsessive thoughts is to ignore them and pay no attention to them. Your prayer is valid as long as it is merely a doubt, because doubt concerning an act of worship after its completion does not affect its validity. The ritual bath for menstruation is sufficient for major ritual impurity (janabah), and you are not required to perform a ghusl for embracing Islam because you are already a Muslim. What is happening is merely an instigation from Satan.