How can one overcome the desire for isolation and death, focus on studying, and regain passion for it, while maintaining prayer performance in the mosque?

We advise you to persevere in seeking knowledge, remain steadfast in obedience, and avoid bad company. Seek out righteous companions who will help you in this. Be diligent in praying in the mosque, while maintaining daily adhkar (remembrances) and Quran recitation. There is no harm in some permissible recreation when you are tired from studying. If your female cousin is not a mahram (definition) to you and you can marry her, then that is preferable. Otherwise, turn your heart away from her and focus on attaining high ranks in this world and the hereafter through study, growth in faith, and righteous deeds.