If one day I feel what love might be, in one thing I'm fully certain, that it would be less than what I feel for my son Nilolas.
Remarks about the translation
maybe I got now the english grammar. the portuguese grammar always have understood.
"era" is the portuguese "past-time".
He/she said, that in the future will understand what love was in his/her past, but only knows, that he/she loves his/her son more than anything what can be described by "love". - This is the context explained.
But maybe it should be still better, to interprete the part of text like: "If one day I'll feel what love would be, in one..."
Is the verb "era" in the past? This doesn't really make sense in English. Also the verb "feel" should be in the simple present tense in both cases, I think. Also, your earlier translation, "I am fully certain" was better than what you have now. If you make these changes, I'll accept it.