Sia is talking about maltreatment in this song. There are many phrases that proof that.
"I've got a thick skin and an elastic heart" means that all the insults and blows she receives don't affect her. Her thick skin makes them to "rebound" instead of affecting her. But this isn't true, because they actually hurt her, but she doesn't show her pain to anyone.
And when she's saying that she's got an elastic heart, means that she has to "adapt" to love and to her lovers. Her heart is a metaphore of her love. And her love is elastic, meaning that she also has to be the one who forgives to the other one, and the one who gets hurt.
"I'm still fighting for peace" means that she makes all the possible she can do for not fighting. She doesn't like fights, she wants to live in peace.
"I'm like a rubber band until you pull too hard" means that by the moment she's holding on. She's like a rubber band because she changes of appearance. The inults hurt her, but she has to pretend like she's strong, when actually she isn't. She's forgiving to the others. When the others "pull too hard" it will be the moment that they run down Sia's patience, and it will be the moment that she will "exploit" and do some insanity.
"Oh why can I not conquer love". Sia explains that when she loves someone, she's so sure that their love is true. But it isn't, because at the end all her lovers hurt her. She wants to "conquer love", because she wants a complete real love, with peace and without fights.
"But you won't see me fall apart", means that even though the insults hurt her, she will never show her pain to the others. She saves her pain for herself. By saying "falling apart", Sia refers to cry.
"And I might have thought that we were one". She refers that she thought that their love (of her and her lover) was true. But it wasn't, because her lover hurted her.
"Wanted to fight this war without weapons". The weapons are a metaphore of the insults and blows she receives. SHe explains that, even though she receives them, she doesn't want to do that, because she's not like this. She's a peaceful person.
"And I wanted it, I wanted it so bad, but there were so many red flags": what she wanted so bad was to have a real love. The red flags are a metaphore of the scars left on her body because of the blows she receives, and the scars left on her self-esteem because of the insults she receives.
"And another one bites the dust" refers to the moments when she's hitted and she falls in the floor. She's "biting the dust when she falls in the floor.
"Yeah let's be clear I trust no one" refers that she's been hurted so many times that, actually, she trusts no one, because she has realized that everyone can hurt her easily, because she's a sensitive person that gets hurts easily.
I could carry on explaining many other examples, but I think I've been clear.
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