You just described the person Jesus talked about in Matthew 7:21: “Not everyone who calls out to me, ‘Lord! Lord!’ will enter the Kingdom of Heaven. Only those who actually do the will of my Father in heaven will enter." Lots of people say the sinner's prayer and think they are saved. Many are not. If all it took was a simple, insincere prayer then it would be the wide path that leads to the Kingdom, not the narrow one. And there are others who are truly saved but fall in some way later on. Ain't none of us perfect, after all. The indwelt Holy Spirit will convict that person of the need to repent - like David when confronted about his sin with Bathsheba. With sexual sins in particular, one can ignore the Spirit and keep sinning, but the cognitive dissonance involved would be terrible to bear. That person, if truly saved, still gets to Heaven, but will have a horribly unhappy life. Someone who hedonistically keeps sinning without a care in the world was never saved in the first place.
There’s grace, and then there’s judgment. Paul makes that very clear. “The Lord shall judge His people” (and that isn’t at the microsecond of grace)
Hebrews 10:
26For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28He that despised Moses’ law died without mercy under two or three witnesses: 29Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? 30For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. 31It is a fearful thing to fall into the hands of the living God.