Why does the focus system change the verb affix just because I bought something?

I’m a grammar-focused learner trying to map out why 'bumili' vs 'binili' feels like a trap. I understand that the actor focus changes the meaning of the sentence entirely, but when I’m actually trying to speak Tagalog, I freeze up trying to decide which affix to attach to the root. Does anyone have a mental shortcut for deciding the focus before the rest of the sentence falls apart?