What is the remedy for obsessive-compulsive disorder (OCD) regarding intention, which leads to doubting the validity of acts of worship, and what is the ruling on water entering the vagina unintentionally during ghusl?

The treatment for obsessive thoughts (waswas) is to disregard and ignore them. When you intend to perform ablution (wudu) or prayer (salat), bring the intention to your heart and begin immediately. Ignore any intrusive thoughts suggesting that you did not intend correctly or at all. If you struggle against these thoughts, Allah will remove them from you. Abandoning prayer is a grave sin, and one must repent from it. There is no harm for you in acting upon the opinion of those who do not oblige you to make up missed prayers, as long as you are afflicted by obsessive thoughts. As for water entering the private parts during ghusl (ritual bath), this is among the obsessive thoughts that must be ignored and given no attention.