What is the way to sincere repentance and to distance oneself from forbidden relationships, after these relationships have been repeated and a feeling of loss has set in, with the beginning of the blessed month of Ramadan?

You have done well by deciding to repent and return to Allah and abandon His disobedience; it is the best decision in your life. To remain steadfast in this decision and for your repentance to be sincere (tawbah (definition) nasuh), you must strengthen your faith and increase your knowledge of Allah, for whoever knows Allah will fear Him. What will benefit you in this is reciting the Quran, contemplating its meanings, and acting upon it, reading the Prophet's hadith (definition) and the biographies of righteous people and repentant ones, accompanying righteous people in real life, and abandoning bad company and means of temptation and corruption. For purification comes before adornment, and your past experience should be a lesson to you regarding the dire consequences of sin and losing companionship.