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Quote If a priest said that swearing is a mortal sin, then I would stop swearing and confess my mortal sin. No big deal ...

"Kill them all, let God sort them out"

Yes, it is a mortal sin. Ugly vile words that are blasphemous are against the Second Commandment. Foul words that are impure are against the Sixth Commandment.  

I encourage everyone to read the whole sermon of St. Alphonsus posted in the OP. I only quoted the first and last portions for brevity's sake. St. Alphonsus gives clear authorities from Scripture and clear reasons why swearing is as such a mortal sin that, if not repented of, will lead to hell.

Mark in para 9 the words, "when immodest words come to the tongue, you must suppress them; otherwise, by uttering them, you shall inflict on your own soul, and on the souls of others, a mortal and incurable wound."

In para 8, "God punished both of them: one became blind, and a sore broke out on the other, which produced deadly spasms. Henry Gragerman relates (in Magn. Spec., dist. 9, ex. 58), that one of those obscene talkers died suddenly and without repentance, and that he was afterwards seen in hell tearing his tongue in pieces; and when it was restored he began again to lacerate it."

In para 7, "O God! how great, were it in their power to weep, would be the wailing of the angel-guardians of these poor children that are scandalized and brought to hell by the language of obscene tongues!"

In para 6, "Besides, by using immodest language, you expose yourself to the proximate danger of falling into unchaste actions: for, according to St. Jerome, as we have already said, ”he who delights in words is not far from the act." etc etc etc.

Besides, those who swear frequently will find themselves falling into all kinds of other vices they will struggle to remove, unless they remove the bad practice of using indecent words at its root. When they come to mind, as St. Alphonsus says in para 9, suppress them. St. Alphonsus also cites St. Paul in Sacred Scripture there, "”But, ” says St. Paul, “fornication and all uncleanness, let it not so much as be named among you, as becometh saints.” (Ephes. v. 3.) Mark the words”all uncleanness."

And while it is an admirable sentiment that you would stop swearing if a Priest told you to, do be a little careful on the "no big deal" part. The early Church, as St. Louis Montfort witnesses, used to stipulate 7 to 10 to 15 years of penance for each single mortal sin:

"Now these Canons exacted seven and sometimes ten or fifteen years' penance for one single mortal sin, so that a person who was guilty of twenty mortal sins would probably have had to perform a seven year penance at least twenty times, and so on." http://www.catholictradition.org/Classics/secret-rosary49.htm

Much much easier and better for all of us to entirely avoid the habit altogether, I think, than, having gotten into it, to hope not to have to make much painful reparation for it later on, either in this life, or what may perhaps be even more long and rigorous, in Purgatory.

God Bless.