Is it permissible to work in a bank in the field of marketing loans, knowing that I cannot find another job and do not want to wrong my fiancée by being unable to complete the marriage?

Working in usurious banks is unlawful (haram), and working in the marketing of usurious loans is even more unlawful due to its nature of inviting to and cooperating in usury, which is one of the gravest major sins. If the last offer is for marketing usurious loans, then it is not permissible to accept it.

As for your wife: If she is distressed and wishes for a divorce, then you must divorce her, for she has the right to request separation due to your inability to provide the dower (mahr) and maintenance (nafaqah). However, if she patiently endures with you until you find lawful earnings, then there is no need for divorce.

You must strive to search for a permissible job, seek Allah's help, and frequently supplicate Him, while having good expectations of Him and relying upon Him. "And whoever fears Allah – He will make for him a way out and will provide for him from where he does not expect."