Will God forgive someone who touched his fiancée, shook hands with her, and engaged in sexual activity short of intercourse with her before the legal marriage contract, and will this negatively affect their relationship after the contract?

The door of repentance is open, and Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" [Az-Zumar: 53], and "O you who have believed, repent to Allah with sincere repentance." [At-Tahrim: 8]. The Prophet, peace be upon him, said: "The one who repents from sin is like one who has no sin." The conditions for repentance are: abstaining from the sin, regretting it, and resolving not to return to it. If the sin involves the rights of another human being, then it is also required to return what was wrongfully taken or obtain their forgiveness. One must hasten to repent before its door closes, which will be when the sun rises from the west or at the moment of death (gharqarah), for Allah Almighty says: "The Day some of the signs of your Lord come, no soul will benefit from its faith" [Al-An'am: 158], and the Prophet, peace be upon him, said: "Indeed, Allah accepts the repentance of His servant as long as he has not ghargharah (reached the rattling of death)."