Yes, I agree it was a chance for Antlers to maybe be proactive and offer Mother's Day to his W, but put yourself in his shoes. His W recently brutalized him verbally really knocked him for a loop. He has been dark since. For her to expect him to come back from that with a kind and thoughtful offer is expecting an awful lot!
In many ways, I like that he didn't proactively offer that up. If he had, what would that show her? That she can treat him poorly and yet still have him extend himself to her? Why didn't she ask? Because she wouldn't suck up her pride and ask, that's why. She doesn't want to ask him for anything, she just wants to be angry. Well, that's her choice.
Antlers, this is just my two cents, don't take it as gospel, but I don't think you should use the words "sorry" or "apologize" when you reply to her, nor should you "explain" why you messed up, because you didn't. She set the current tone of your relationship, which is that she wants nothing to do with you. Oh, except when there's something special she wants, and then she expects you to offer it up to her? I know you feel majorly at fault in your M, and that may be true, so you want to show your W you love her and want to be kind to her, but replying back to her with your head down and tail between your legs will only make her lose more respect for you. Be nice and set a more positive tone for the future.