Seeking forgiveness is compulsory to avoid the dangerous consequences of the sin. Beside its necessity to repair the damage of the sin, it gives a feeling of relief and comfort to the sinner because it is part of remembering Allah which is the only source of peace of mind (Sura 13, Verse 28).
Nevertheless, it does not mean definitely that Allah (SWT) Has forgiven the sin. Allah forgives our sins if we sincerely seek forgiveness and decide not to do it again, and give back the rights of others, but He knows when?
We need always to remember our previous sinful acts and keep on seeking forgiveness.