Nicole,
I think I recall you saying you have a Catechism, if so look up “sin” in the index, it should direct you to paragraphs that describe mortal and venial sin. I’ve found that the Catechism is not only the place to find the authoritative teaching of the Chruch, it’s very well written.
Here’s a link:
http://www.usccb.org/catechism/text/pt3sect1chpt1art8.htm
Mortal sin* destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. *
Venial sin allows charity to subsist, even though it offends and wounds it.
Your parish catholic bookstore may have available a small pamphlet on the examination of conscience; in it you can find a list of various mortal and venial sins. It won’t be all-inclusive but it will help you understand the difference between them. I’m sure that you can quickly find links to similar lists.
Mortal sin requires the Sacrament of Reconciliation before receiving the Eucharist, in fact, if we don’t go to confession and receive absolution first we commit another mortal sin by receiving.
Venial sins are adequately confessed during the general confession at the start of Mass. We don’t have to confess venial sins in the confessional, but in the struggle for sanctification it can be very valuable to do so. I’ve been struggling with this myself, I tend to scrupulosity and I got a lot of good insight from Benedict Bauer’s book
Frequent Confession.
If we make a good confession and receive absolution for a mortal sin, it’s forgiven - period. It still remains a fact that happened, something that we remember with regret and intend to never repeat (by God’s grace), but that’s not the same as needing to confess it again.
My understanding is that honestly forgetting something in the confessional doesn’t invalidate your absolution. If you remember it later, then confess it the next time you go. At my first confession, I had to confess to having comitted a number of mortal sins dozens (or many hundreds) of times, so many that I couldn’t possibly remember every incident. My confession was not invalid, and the absolution extended to all of the sins of that kind because it was my intent to confess them all.
Remember, it is not a sin to have disordered thoughts come into our heads. Push them away, say a prayer, renew the act of your will in which you reject sin and turn toward God - and you’ve accepted God’s grace and united your suffering to Christ’s. This pleases Him.