A reader struggles with Purgatory and Mortal/Venial Sin Distinctions

He writes:
I must admit that I’m struggling with reconciling some things I’m reading about sin and purgatory. I can see the need for a time between death and heaven in terms of finishing the work that Christ has begun before we are fit for heaven. I can embrace the idea of a place were our loving Father finishes his work of gently transforming his children. However, some of the things I’ve read by Catholics present it as something very close to torture. One website said, “Expect it to be brutal”. A number of people also seem to relate it to particular sins we’ve committed, rather than the flaws in us that still need to be healed. They make it sound an awful lot like being punished for sin and that doesn’t feel compatible with the forgiveness that God offers in Christ. It makes me wonder how anyone could face death with anything but sheer terror! That doesn’t resonate well with the hope I read in scripture.
For my take on purgatory, go here. There are a couple of pieces there. As far as the pain of purgatory, I think the analogy of the “runner’s high” is useful. Purgatory, like all change and growth, hurts. But it’s a good hurt. So some saint describe purgatory’s pains as worse than any earthly suffering while other describe purgatory’s pleasures as more exquisite than any early pleasure. This is not different than Paul’s point: suffering produces perseverance, perseverance produces character, and character, hope . For the love of God has been poured out in our hearts by the Holy Spirit. (Romans 5:1-5) Likewise, Hebrews 12:5-11 speaks of suffering as a form of fatherly discipline: Pain unto life, not pain unto death. Purgatory basically is the teaching that what we experience here in the process of