The trouble is that both words have a verb sense and a noun sense.
The noun sense of affect is something like “mood” or “emotion” and isn’t used often, while the noun sense of effect is “thing that happened (because of some cause)” and is a rather common word.
The verb sense of affect is “to cause something to happen (to something)” and is a pretty common word, while the verb sense of effect is more like “to make something be true” as in “effecting change” above.
The mnemonic I use is from dungeons and dragons, some spells are “mind-affecting effects” meaning they change minds and they’re caused by the spell being cast.