According to the Catholic Church, can having ideations to sin ever be mortally sinful? In the Catholic Church, is having ideations to sin a mortal sin? In other words, would a faithful Catholic have to repent during Confession for having ideations to sin or having attempted, but failed, to act on such ideations?
"Ideations" in the question "Can having 'ideations' to sin ever be mortally sinful?" could mean "passions of the soul," which are causes of sin.

Now, can a sin committed through passion ever be mortal? St. Thomas Aquinas explains (Summa Theologica I-II q. 77 a. 8 c.):


  Mortal sin, as stated above (q. 72, a. 5), consists in turning away from our last end which is God, which aversion pertains to the deliberating reason, whose function it is also to direct towards the end. Therefore that which is contrary to the last end can happen not to be a mortal sin, only when the deliberating reason is unable to come to the rescue, which is the case in sudden movements. Now when anyone proceeds from passion to a sinful act, or to a deliberate consent, this does not happen suddenly: and so the deliberating reason can come to the rescue here, since it can drive the passion away, or at least prevent it from having its effect, as stated above: wherefore if it does not come to the rescue, there is a mortal sin; and it is thus, as we see, that many murders and adulteries are committed through passion.


So, if your reason tells you committing a mortal sin is bad, yet you consent to it or actually commit it anyways, this itself is a mortal sin.