What is your advice for expanding your horizons, when your world has become small?

Pareto's rule. You want 80% of your life to be order, and you can use the strength and stability of that to tread into the unknown as 20% of what you are exposed to.

I'll use my current progress as an example. I do a LOT of college and university courses. Once I've done several in one area, I'll then move into certification in another area that has a lot of crossover with what I've already done, and then once I've got quite a few credentials in that area I'll move to another and by that point I'm thinking "I've covered much of this topic in a previous course, I just need to apply a some new details within this new context".

And try to monetize the domains you move into. If you are able to, then that will be a concrete, objective proof to yourself and others that you have effectively mastered that domain. If someone is willing to pay you for your work then that is telling you "your contribution in this area is so great that I am willing to contribute some of my limited resources and time to access it".