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.
Wearing Niqaab (covering face) is a religiously desirable act in Islam without any disagreement among the scholars. However, they disagreed about its being an obligation; see the details in Fataawa 85816 and 92400.
If the enquiring sister believes that wearing Niqaab is just a desirable matter, not an obligation i.e. there is no sin in not wearing it, then we hope that she will not be sinful if she does so based on her conviction of the opinion of the scholars who permitted exposing the face. But it is better for her to wear Niqaab since many of those who are of the opinion that wearing Niqaab is only desirable, make wearing it obligatory at the time of temptation and immorality.
On the other hand, if she believes that wearing Niqaab is an obligation then she commits a sin if she does not observer it.
As for the question “Will Allaah forgive me…?” know that Allaah is Most Forgiving and Most Merciful, and He accepts the repentance of a slave whenever one turns back to Him and makes sincere repentance fulfilling the conditions of a sincere repentance mentioned in Fatwa 83145. Allaah Says (what means): {But whoever repents after his wrongdoing and reforms, indeed, Allaah will turn to him in forgiveness. Indeed, Allaah is Forgiving and Merciful.}[Quran 5:39]
The Prophet, sallallaahu ‘alayhi wa sallam, said: “A person who repents from the sins he has committed is like a person who has not committed a sin." [Ibn Maajah]
Allaah Knows best.