Is intense thinking about and longing for friends and teachers, to the point of thinking about them during prayer, considered a sin for which one will be punished by Allah, and is repeating this sin after repenting from it considered a trivialization of that repentance?

The love described in the question is closer to infatuation. If it occurred without cause, then one must strive to divert their thoughts from these friends and reduce communication with them, and occupy themselves with what brings good in religious and worldly matters. The love of Allah consumes the love of all else. The saying: "Whoever loves something other than Allah will be tormented by it" means torment of the heart. For infatuation, even if its owner finds pleasure in it, is among the greatest torments of the heart. Among the conditions for a sound repentance is the firm resolve not to return to the sin, but sinning again does not invalidate the previous repentance.