Clearly the mercy of God is immense, beyond measuring. He is ready at any time and in any place to forgive literally anybody for having done literally anything. But, that said, it also appears that God's forgiveness requires something on our part – sorrow for our sin.
To be genuine, moreover, sorrow for sin also requires something else. Usually that it is called a "firm purpose of amendment" – the determination not to sin again.
Obviously this isn't certainty of not repeating one's sin, for who can guarantee that? Rather, a firm purpose of amendment is the honest intention to make a serious, sustained effort not to sin again. And that intention must include determination to begin the effort here and now, not to delay until some point in the future when it may be easier nor to proceed a little at a time according to somebody's notion of "gradualness."
The account in chapter 8 of John's gospel of the woman taken in adultery, frequently cited in discussions of these matters, is a good example. Yes, in this moving and dramatic episode Jesus does indeed extend mercy to a sinner. But he also tells the woman, "Go and sin no more."
He doesn't say cut back a bit or put it off until it's convenient. He says, "Sin no more."
But, someone might object, isn't the conversion of St. Paul an instance of God's mercy reaching out to its object even before sorrow and a purpose of amendment were present in Saul, who at that time was a furious persecutor of Christ's followers?
Yes, it is. But note that in persecuting those early Christians, Saul of Tarsus truly believed he was doing the right thing. His very zeal, mistaken though it was, helped draw God's mercy to him. The first letter to Timothy explains: "I obtained the mercy of God because I acted ignorantly, in unbelief" (1 Tm 1.13). The pre-conversion Paul was wrong but he honestly thought that he was right. God stepped in mercifully but forcefully to correct that mistake.