I've been Catholic all my life-- being raised Catholic, you get a very different viewpoint on the faith than someone who comes into the faith as an adult. I sponsored my GF Nikkie when she became Catholic three years ago. We took intensive classes on the church, and everyone was invited to ask any question they had. Our priest teaching the class was open and honest and answered everyone's questions as best as he could. This was important, because there is so much misinformation about the church out there. As best as I understand, here is the modern church's teaching:
For sin to be present, the following two conditions must be met:
1. The person must know the act is a sin, and
2. The person must commit the sin willingly, with intention.
Therefore, suicide may not be (and 99.9% of the time probably is not) a sin, because someone who commits suicide usually has some kind of mental illness that "caused" them to end their life-- therefore, they did not do it willingly, with full intention.
Also, the Catholic church teaches that we can never judge who is in heaven or hell, because we are mere mortals and don't understand everything about an individual's life. The judgement is left up to God-- not us, priests, or even the Pope.
Also, I wanted to clear up a bit about mortal sin. Mortal sin is not "unforgivable". It's not really an action either-- more like a state of being. It simply means that the person has closed themselves off from God. Someone who has commited a mortal sin can turn back to God and be forgiven.
Hopefully this helps a bit. If you have any other questions about the Catholic Church, I'll be glad to answer them. I'm not an expert, but I can always call on my brother (a Catholic youth minister with majors in philosophy and religious studies-- way smarter than me.

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