Why does the partitive case feel so inconsistent?

I've been studying Estonian grammar for three months, and while I grasp the basics, the partitive case keeps tripping me up when I try to form sentences on the fly. I understand the rule for vowel stems, but I'm struggling to internalize when to use it versus the total object in natural conversation. Does anyone have a mental shortcut for shifting from formal grammar rules to intuitive speech without pausing to calculate the inflection every time?