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Here is a link to your question on Stack Overflow: https://stackoverflow.com/questions/56855258/ggplot-confidence-intervals-too-narrow-to-see
For help on making a reproducible example, see our FAQ here: FAQ: What's a reproducible example (`reprex`) and how do I do one?
As I said in a comment on Stack Overflow, you need to calculate your CI from your model. Right now you are having geom_smooth() calculate the standard error based on a linear model fit to your predictions with lm(). Ideally ggpredict() will calculate the CI for you (I haven't checked the documentation), which should make things a little more straightforward.
I show a basic example of adding CI calculated from the model with geom_ribbon() in this section of a blog post, which may help demonstrate the concept.