Ja nigdy nie przesta≥am ciÍ kochaś. Nawet przez sekundÍ.
Both are correct but in this particular sentence most Poles would say "na".
"Przez" is definitely correct and more elegant, whereas "na" tends to be colloqial in this context. IMHO.
Perhaps more Poles would prefer 'na' against 'przez' here, perhaps not. My view is most of them would say 'przez' in this particular couple of sentences translated by Wulkan.