What is the expiation for the repeated crime of fornication with a number of people, where the relationship was semi-complete (external ejaculation without penetration), and how does one recognize accepted repentance?

Lenience with non-mahram men leads to temptation (fitna), for Allah Almighty said: "O you who have believed, do not follow the footsteps of Satan. And whoever follows the footsteps of Satan - indeed, he enjoins immorality and wrongdoing," and the Messenger of Allah, peace and blessings be upon him, said: "No man is ever alone with a woman but the third of them is Satan." What happened is an approximation of adultery, and its expiation is sincere repentance by abandoning the sin, regretting it, resolving not to return to it, and avoiding temptation.

The signs of accepted repentance, as mentioned by Ibn al-Qayyim, are that the state of the repentant person is better than it was before repentance, that the fear of Allah continuously accompanies him, and that his heart is filled with regret and fear commensurate with the gravity of the transgression.

The repentant person must feel the vastness of Allah's mercy, for no sin is too great in comparison to it. He is the One who said: "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.'" And the Messenger of Allah, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin."