Intent to confess is not sufficient. The Council of Trent and the ordinary universal Magisterium infallibly teach that you must actually confess every actual mortal sin and the number of times it was committed, as best you can remember:
The Council of Trent:
“all the mortal sins, of which, after a diligent examination of themselves, they are conscious, must needs be by penitents enumerated in confession”
“the faithful of Christ are careful to confess all the sins which occur to their memory, they without doubt lay them all bare before the mercy of God to be pardoned”
“If any one saith, that, in the sacrament of Penance, it is not necessary, of divine right, for the remission of sins, to confess all and singular the mortal sins which after due and diligent previous meditation are remembered… let him be anathema”
The CCC 1456
“Confession to a priest is an essential part of the sacrament of Penance: “All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession”
If it is an actual mortal sin, you must confess it to a priest in the confessional. The intent to confess is not sufficient. . The Magisterium does not teach that mere intent to confess makes for a valid confession.
Don’t give long explanations in the confessional, especially if you have many sins to confess. First confess your actual mortal sins, and the number of times committed. This should not take very long. Then you can confess some venial sins, and perhaps ask a question or add a brief explanation.