If somebody has practiced Zena and is in serious repentance and thinking herself the biggest culprit, then what measures should she take for peace and relaxation? Will Allah forgive the person? And also the measures for seeking closeness with Allah?

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Do sincere isteghfar and be in the state of wudhu all the time to keep yourself away from evil thoughts. And pray namaz at prime time. Be in the company of good practicing God fearing momenenaat .
Read Quran twice a day with translation and aayat no 28 from surah Raad.