I’m not a Catholic either, but I’d love to see any kind of scriptural justification for the teaching of purgatory. From my perspective, I fail to see how purgatory can do anything BUT instill the idea that Jesus’ sacrifice on the cross was insufficient.
"Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ’s afflictions for the sake of his body, that is, the Church"
What did St Paul mean by this? It can only be a reference to that part of Christ's passion which can indeed be lacking - that part where we join ourselves to it, become one with it - or where we fail to do so.
Most people die neither fully connected to God, not fully at war with Him. Purgatory is simply the passage of the soul to full communion with God: the changing of an impure, damaged soul into a pure, whole, sinless one.
That passage, that process will be hard, indeed many of us will find it a terrible ordeal. But it will not last for ever, and then we shall be safe with Him, the true author of our being.