Is it valid to seek forgiveness despite not abandoning sins, not regretting them, and expecting to return to them in the future?

You must engage in sincere repentance (Tawbah Nasuh) for all sins, fulfilling its conditions: immediately ceasing the sin, resolving never to return to it, and regretting what has passed. What you feel about returning to sins is from Satan, to deter you from repentance, so do not surrender to him. Even if you return to a sin due to your weakness, return and repent and do not tire. As for seeking forgiveness with the tongue while persisting in disobedience, that is an incorrect repentance. So strive to make your repentance sincerely for Allah and to remain steadfast on His law.