when people speak, they are mostly talking about themselves. If your spouse is saying "you don't care. You're heartless", (s)he most likely means to convey "I don't feel loved enough".
I would suggest The 5 Love Languages and a frank discussion about it. Before that you may need to honestly assess the relationship though - are you putting in effort? Do you do/say less romantic things than you used to? Or is (s)he expecting too much from you?
I would stop apologizing if you aren't sincere, that doesn't help either of you. A deep breath and the honest truth will get you a lot farther than placating her, that only postpones the argument.
