Have you got a copy of the Cathechism? That might help. If you don't have a paper copy, it's on the Vatican website.
As I understand it, your questions are on a) purgatory and b) guilt-joy. Do correct me if wrong. I'll give a brief(ish) answer to each, so far as I can (some have already received very good answers), but the CCC (Catechism) link will point you to relevant Scripture passages to back it up. I should be writing an essay so I don't want to give you the full 5000 word version with full Bible quotes and all as I really must get down to work.
But let's give it a go.
Purgatory
Catholics teach that there are two punishments due to sin: temporal and eternal. The debt of the eternal punishment was paid once and for all by Jesus on the cross, it is what allows people to enter heaven and avoid hell. The temporal punishment may still remain. The analogy often given is that of e.g. a child who gets angry and throws a stone through a window of their parents’ house. The parents may forgive the child when s/he says sorry and the family are reunited in love, but they’ve still got a broken window which needs to be repaired and it may be appropriate for the child to contribute to that out of their pocket money, for example. Also, the child will want to show their contrition and love for the parents and so may make extra efforts e.g. to make a cup of tea, write a card etc. Eventually the window will be repaired and they’ll all put it behind them (thus it is temporal, temporary).
What happens when you die is that the particular judgement is immediate. Either heaven or hell, there are no other options in eternity. For those who are destined for heaven, however, it may be that there is still a temporal debt to pay (it may not, it is perfectly possible to unite oneself in charity to God so that the debt has already been paid during one’s life) so that one needs to be purified before entering heaven. This is purgatory. The soul is purified ready to meet God face to face in heaven.
Our prayers can help people in purgatory in the same way they can help people on earth, because we are all united in Christ as part of the Mystical Body. Christians are united not just because they all have the same goal (to follow Jesus or to be united with God or whatever other shorthand you would use to sum up the whole Christian life) but because they a members of the same body and so strive together to co-operate with Jesus the Head and thus achieve things as a body which the individuals could not.
Joy-Guilt
Catholics teach that eternal life begins now by being united with God and we don’t have an abrupt cut off point at death. So in a way it is very joyful. We also teach that there is nothing that can separate us from God except our own decision to reject him and there is no sin which is unforgiveable, so again a cause for joy in God’s mercy. And confession is an incredibly liberating experience because not only do we confess our guilt but we receive God’s forgiveness with absolute certainty, there is no doubt that we have been freed from our sins forever. You never feel guilty for long! But overall there is a deep joy and peace which comes from being with God. The guilt we do feel is suitable in order to make us realise the seriousness of sin. But there’s no need (and the Church certainly doesn’t teach) that we should feel guilty for not living up to an impossible standard. We don’t have to ‘earn’ God’s love or mercy, which are freely given. We can merit additional graces to help our own holiness and to help build up the body of Christ, but these make us better saints and it’s more like reaching our full potential than a weighing scales of good vs bad ‘works’. The picture they give is of a series of jugs full of water. A 200ml jug can be full and a 2litre jug can be full. One is not ‘fuller’ than the other but one does contain more than the other. So we can become more ‘full’ of grace not by an absolute measure of quantity, since grace is infinite, but by increasing our capacity to receive God’s grace within us, principally by charity. All the works begin and end in charity and ‘dead works’ (works done without love) are useless. So it’s not so much what you do as the love with which you do it.
Sorry that's all I've got time for now, but I hope some of it might help.
Disclaimer: I'm writing this as a Catholic for the benefit of someone who's interested in becoming Catholic. Obviously these are beliefs not scientifically provable 'facts' so when I say 'X is the case' I am not claiming that everyone in the world/the majority of people on MN agree with X, clearly not. These are truth-claims which make sense within the whole picture of faith, not absolute claims to a definitive understanding of what everything is all about.