Is my fasting and prayer valid, knowing that I performed ghusl at suhoor with the intention of ending my menstrual cycle, and I fasted and prayed, and nothing (no discharge) came down until Maghrib, then I confirmed its (the cycle's) end?

It is not permissible for a woman to perform ghusl (ritual purification) from menstruation, pray, or fast before being certain of its cessation. A woman ascertains this by the emission of a white discharge (al-Qassah al-Bayda') or by the dryness of blood. Women have unanimously agreed on this, just as the women among the Companions used to do. If she is certain of purity before dawn, it is permissible for her to intend to fast, but she must make up the day if she performed ghusl before being truly certain of purity.