Stuck on the case system: When should I actually use the instrumental case in daily speech?

I’ve been learning Bosnian for a while now, but I feel like I'm hitting a wall with the cases, specifically the instrumental. I know the grammar tables, but when I’m talking to my partner, I find myself defaulting to the nominative just because it’s easier. How do I move from 'knowing' the cases to actually using them correctly without thinking for five seconds before every sentence?