Can "feelings and passions" make an act involuntary? In high school, my religion teacher said,


  Mortal sin is a very serious matter. It is a conscious decision in which the  person intentionally turns away from God. Therefore it is impossible to, say, attend Mass on Sunday in the state of grace, commit a mortal sin during the week, and attend Mass the next Sunday having already repented.


On the other hand, it seems entirely possible for a person to be a devout Catholic, attend Mass, then fall to temptation one night, watch pornography, masturbate, feel remorse immediately, and repent. A priest even advised me to go to confession immediately the next day whenever this happens.

Was a mortal sin committed in this case? Or is this a case where


  ...the promptings of feelings and passions diminish the voluntary and free character of the offense...


(CCC 1860)

?
Is someone strapping you to a chair, propping your eye's open, and forcing you to watch pornography? If not, then you certainly have freely chosen to watch it and doing so certainly is a mortal sin because:


Pornography constitutes grave matter.The sex slave / pornography industry is incredibly inhumane; giving material support to it is itself a sin.
You voluntarily watched it.Just because you had "feelings and passions" does not mean your act was involuntary. For example, if you become drunk, drive, and kill someone and don't even remember what you did, you are still responsible for having indirectly willed murder.For more info on indirectly willed acts, see §94 of Moral Theology by McHugh &amp; Callan, O.P.Also, concupiscence (the body rebelling against the soul, "feelings and passions" inclining you to act against the dictates of reason) does not cause involuntariness, as St. Thomas Aquinas argues in Summa Theologica I-II q. 6 a. 6 co.:Concupiscence does not cause involuntariness, but on the contrary makes something to be voluntary. For a thing is said to be voluntary, from the fact that the will is moved to it. Now concupiscence inclines the will to desire the object of concupiscence. Therefore the effect of concupiscence is to make something to be voluntary rather than involuntary.Note that CCC 1860 is not claiming that "feelings and passions" make an act involuntary.(cf. also St. Thomas's question "Whether ignorance causes involuntariness?".)
You know it's an offense against God:St. Matthew 5:28-29:…whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart. And if thy right eye scandalize thee, pluck it out and cast it from thee. For it is expedient for thee that one of thy members should perish, rather than thy whole body be cast into hell.


Lust is a capital sin because other sins (like masturbation in your case) follow from it. Masturbation is an intrinsic evil. Onan "spilled his seed upon the ground" (Gen. 38:9) "And therefore the Lord slew him, because he did a detestable thing" (Gen. 38:10).

The Doctor of the Church St. Thomas Aquinas writes that masturbation is an "unnatural vice" (Summa Theologica II-II q. 154 a. 11 co.)—a type of lust like bestiality, sodomy, and pederasty—and that, as in contraception,


  procuring pollution [i.e., ejaculation], without any copulation, for the sake of venereal pleasure … pertains to the sin of 'uncleanness' which some call 'effeminacy'*


*Latin: mollitiem, lit. 'softness, unmanliness'

Pray the rosary (at least 5 decades) daily. This is an excellent remedy against sins of impurity.See


Part 1, "4th Decade: Marvellous Effects," of St. Grignion de Montfort's The Secret of the Rosary, which describes how the rosary has converted hardened sinners (including those who prior to praying it were immersed in sins of impurity, like Bl. Alan de la Roche).
the stories of St. Alphonsus of Liguori's The Glories of Mary.


Marian devotion has a long history of helping those struggling with sins of impurity.