How can one get rid of anxiety regarding future studies and livelihood, and strengthen the conviction that God is the Provider and the Doer of good?

A person must differentiate between the means (causes) and their Causer. Allah is the Provider and the Ordainer of the means. Focusing solely on the means is polytheism (shirk (definition)) and a deficiency in intellect, whereas completely abandoning them is a disparagement of the Shari'ah. Reliance upon Allah (tawakkul (definition)) means depending on Him while utilizing lawful means. Allah is capable of ordaining multiple means for sustenance. "And whoever fears Allah – He will make for him a way out and will provide for him from where he does not expect." (Quran 65:2-3) "And whoever relies upon Allah – then He is sufficient for him." (Quran 65:3) Furthermore, anxiety and worry may stem from sins, and repentance from them is a fortress that removes fear and brings security.