What is the ruling on the prayer of someone who abandoned prayer due to obsessive-compulsive disorder (OCD), then returned to it without repeating ghusl?

The cure for obsessive whispers (waswas) is to ignore them and pay them no mind. Missing prayers is a grave sin that must not be committed. You should have prayed according to your condition and not surrendered to the obsessive whisper about the return of menstruation. As long as you were certain of purity, menstruation does not return except with absolute certainty. You must repent to Allah Almighty and continue to maintain prayer and strive against the obsessive whispers. It is more prudent to make up the prayers you intentionally missed.