Memini can take either an accusative or a genitive, and there is a difference in meaning, but you're presumably modelling this on "memento mori", so let's use an accusative (mori may not particularly look like an accusative but it's an infinitive and infinitives are neuter so the nominative and accusative are identical). Memento is obviously fine: it's a 2nd person singular imperative.
Vita is the correct word to use for life, but its accusative is vitam (vitae actually is the genitive, so a point to machine translation for once): memento vitam
Amorem is indeed the accusative of amor, meaning love: memento amorem
Aeternitas does mean eternity, but its accusative is aeternitatem: memento aeternitatem