Does God forgive and accept the repentance of one who commits fornication with a prior intention of repentance, or does He punish them in this world?

Beware of Satan's whispers regarding sins, for there is no good in disobeying Allah Almighty, and repentance from it is not guaranteed. Stay away from sins, especially major ones like fornication, and marry if you can. The consequences of sins are dire in this world and the hereafter, for every calamity that befalls us is due to what our hands have earned, as Allah Almighty said: "And whatever strikes you of disaster - it is because of what your hands have earned, and He pardons much." (Ash-Shura: 30). Repent to Allah with sincere repentance, get rid of evil thoughts, and ask Allah for guidance and steadfastness.