All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If a person repents while fulfilling the conditions of repentance, then it is hoped that Allaah forgives him; for more benefit on the conditions of repentance, please refer to Fatwa 87903.
Therefore, since this girl repented and regretted what she did, and returned what she had stolen to its owner, then she has to think good of her Lord, and do good in the future. Also, she should ignore these whispers of the devil, that Allaah did not forgive her. How could she know that Allaah did not forgive her, as this is a matter of the unseen about which only Allaah Knows? It might be that the devil wants to lure her so that she will not perform acts of obedience and make her despair of the mercy of Allaah; indeed despairing of the mercy of Allaah is one of the grave major sins. For more benefit, please refer to Fataawa 83145 and 83055.
Furthermore, one is obliged to observe good morals with Allaah and should not object to what Allaah does using such wordings that are mentioned in the question.
As regards the difficulties and hardship that she is experiencing, this does not necessarily mean that they are due to a sin; rather this could be just a test. If she remains patient, Allaah will increase her status; for more benefit on the excellence of patience, please refer to Fataawa 89110 and 83577.
Finally, we advise her not to despair and not to surrender herself to these insinuations. Besides, she should endeavour to treat herself and look for a pious husband, and try to achieve what benefits her in this world and in the Hereafter.
It is confirmed that the Prophet said: "Cherish that which benefits you (in the Hereafter) and seek help from Allaah and do not despair or be lazy." [Muslim]