S'effacer literally means “to erase oneself,” but in the song it means for pain to gradually fade away or disappear. In "Tout ce qui fait mal finit par s'effacer," the singer offers a memorable image of emotional wounds slowly losing their power over time.
"J'assume" is RnBoi owning up to a relationship he knows is bad for both of them, and refusing to pretend otherwise. The title means "I own it," and the hook flips it around: first he tells her "C'que t'aimes, c'est quand c'est toxique, quand on se fait souffrir" [What you love is when it's toxic, when we make each other suffer] and "Assume, assume" [Own it, own it], then admits his half with "J'aime trop quand ça d'vient toxique, quand on se fait souffrir" [I love it too much when it gets toxic, when we make each other suffer] and "J'assume, j'assume" [I own it, I own it]. Nobody here is the innocent one. The coldest line is "J'peux t'traiter comme la reine de mon cœur" [I can treat you like the queen of my heart] followed by "Et j'peux faire tout ça sans jamais t'aimer" [And I can do all that without ever loving you]. That is brutal, and he says it almost gently.
The second half turns into a warning. "Cette fois ne revient pas, protège ton cœur" [This time don't come back, protect your heart] sounds caring, and then he tells her "J'te souhaite d'aller mieux" [I wish you get better] and "D'aller mieux sans moi" [To get better without me]. He even suspects she does not want to heal: "Tu veux qu'on retour en arrière" [You want us to go back]. A tip for learners: notice how much gets squeezed out in sung French. "J'peux" is "je peux", "t'traiter" is "te traiter", and "d'vient" is "devient". Also, "Faut qu't'essayes de pas tomber" [You've got to try not to fall] drops both the "il" of "il faut" and the "ne" of the negation, which is exactly how people talk.