So how can the Christian experience the opposite? How can it be that, in Christ, our life is improving instead of declining?

It sounds crazy, but that's what Paul writes to the Corinthians! That is, as we see Christ more clearly, we share in his glory more and more. It can't be measured biologically or demonstrated scientifically, but it's a very true reality: Life in the Spirit means life, more and more, glorified in the Spirit!