Having one though(t) of a sexual nature is a mortal sin provided that the three conditions for having incurred mortal sin are fulfilled. If you die before confession of the one thought, you will go to hell.
Classicist:
For a sin to be mortal it must:
1. Involve a Grave Matter (like violating the 10 commandments)
2. Done with full knowledge
3. Complete consent
Having a passing impure sexual thought, would probably not fit into the scheme above. There are many things that can diminish ones guilt when one commits a sin even in a grave matter such as external pressures (like the sin has become a habit and as we all know habits are very difficult to break) and psychological disorders. It has been my experience that having a good confessor can help one in these areas to break these disordered habits and that many priests have some knowledge of counseling and psychology and can help. I think your view of things may be skewed due to your underlying problem of OCD. Also, a good confessor can even help you with your problem of scrupulosity and help you to experience the love and mercy of Christ more fully.
The Catechism of the Catholic Church states the following in paragraph 1864:
"There are no limits to the mercy of God, but anyone who refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit."
In other words, God can forgive any sin if we want to be forgiven. Also, if a person were to commit a sin and ask God for forgiveness prior to reaching a confessional (which is something that I do) or even if their intent was to go confession to repent and they die on the way, God in his infinite mercy knows what is in that person's heart and knows their intent and can and will forgive them. There have been many private revelations among the saints such as my patron saint, St Faustina, that indicates that God fights for our soul at the moment of death and allows us to repent even at that late date of all of our sins so that we can spend eternity with our Lord.
Remember that God is infinitely more interested in our salvation than we could ever be. Look at all the trouble He went through to save us. I do not believe that I would have become a slave, like He did, and died such a death, like He did, to save everyone. God's love and mercy is unconditional. It belongs to all of us if we want it.
God's blessings to you,
Jeff S.
www.catholicxjw.com
P.S. If you would like to continue this discussion email me at [email protected]