black lives matter

so i’m white and i’m 14 and my mom raised me to have a good heart, so i bought a black lives matter mask when my family’s constantly making slurs and wanting to say their hot racist when clearly they are, so i walked in to a gas station with my mask on and the cashier says “that should say all lives matter” made me so mad and want to cry and i get back to the car and tell my mom and she says i knew that would happen and i say yeah but this is just a mask they can’t take theirs off. and we argued the whole way back was i in the wrong here?