What should one do who has taken out an usurious loan due to dire circumstances and feels distressed by committing this sin, yet cannot repay the loan all at once?

If you repent sincerely from taking out a usurious loan, then Allah Almighty will accept your repentance and erase your sin, for the one who repents from a sin is like one who has no sin. Repentance is achieved by feeling remorse, abandoning the sin, and resolving not to return to it. If it is easy for you to reduce the installments and usurious interest, then do so. You should frequently seek Allah's forgiveness (Istighfar) and send blessings upon the Prophet, peace be upon him, for these two ease distress and open the doors to provision, by Allah's permission.