What should I do to relieve myself of the burden of money I took from people in exchange for services I did not perform for them, which I cannot return to them or locate them, knowing that I fear my current poverty is due to these debts?

Sincere repentance is not precluded by a servant's inability to return rights to their owners. Rather, he must resolve to return them when possible, even if by giving charity on their behalf. He must also perform many good deeds and ask Allah to relieve him of the burden of these rights. Al-Ghazali said in "Minhaj al-Abidin": If you are unable to return the money due to poverty, or the absence or death of its owner, then give it as charity on his behalf if possible. Otherwise, perform many good deeds and humbly supplicate to Allah to make him pleased with you on the Day of Resurrection.