Not being punished for the sin of zina in this worldly life does not mean one is committing shirk. Also, it is wrong to believe that the sin of committing zina can be only forgiven in case one is given hundred lashes or any other punishment.
If one has been guilty of a sin and Allah has covered for us, then we shouldn’t uncover the veil that Allah has placed over this sin. Rather ask forgiveness of Him and repent to Him sincerely, for He is, above all, all-Compassionate, All-Merciful, All-Forgiving.
No muslim should never lose hope of gaining Allah’s mercy. Sometimes Satan tries to trick the repentant by heaping doubts on them and trying to make them despair of Allah’s mercy. Never pay attention to these whispers and thoughts. What is gone is gone. You have made Tawbah and Allah welcomes those who return to Him. He never let down those who repent sincerely and seek His forgiveness.
In conclusion, after sinning, one should not feel despair. Rather one should forward sincere repentance to Allah and rest assured that Allah Almighty will forgive his/her sin, as long as his/her repentance meets the conditions of sincere repentance. Moreover, you must learn how to forget, for the more you look back to your old days of sinful habit, the wider the door is open for Satan to draw you back. Scholars used to resemble this case to that of gazelle being chased by hound. Despite that gazelle is naturally faster in speed, but its frequent looking back weakens its morale and makes it an easy prey for hounds.