Drowning in cases—how do I keep my cases straight without overthinking in the middle of a sentence?

I’m a busy professional with only 20 minutes a day, and I’m getting nowhere because I stop every three seconds to calculate if I should be using the instrumental or accusative case. It’s ruining my conversation flow. Are there specific mnemonics or 'language chunks' you use to stop the mental grammar calculation so I can just speak Croatian fluently?