What can a mother do, and how can she convince her 19-year-old daughter who has worn the niqab by personal conviction for four years and now wishes to remove it, especially given that the mother views her desire to remove it as a tribulation because there is no clear reason for it?

Your daughter did well by wearing the niqab previously, and you should have encouraged her instead of suggesting its delay. You must now ensure her commitment to her attire, for it is more concealing and purer for her heart, and safer for her honor. Use encouragement and persuasion, acting upon His (Allah's) saying: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones" (Quran 66:6). Covering the face is a matter of differing opinion, but the preponderant view is its obligation. Therefore, show her the fatwas that clarify this, and supplicate frequently for her guidance, for the supplication of a parent is answered.