What's the hardest part about actually using the makeup you already own?

I was thinking about how much harder makeup gets when you have a decent-sized collection.

Not necessarily applying it, but deciding whether a shade actually works for you, which products work well together, whether you already own something similar, what to use when you want a different look, or whether something you're considering buying would actually add anything to your collection.

I'm curious what other people find most frustrating.

If you could have one thing make your makeup collection easier to navigate, what would it be?

And what kind of recommendation would you not trust, even if it sounded confident?