Can relate to what you posted HD. Being a survivor and a supporter is a lot to carry, and sometimes I can't handle it all.
Both he and I have our moments of "cracking" where we can't "be strong" for the other one. And one person's coping mechanisms (e.g. distancing, numbing out) are often the other person's triggers (rejection, abandonment).
His self worth might be so low that he has a hard time seeing that you DO care and you DO get hurt by his actions. Or maybe he's pushing your buttons to elicit a reaction, to "prove" to himself that you care.
As for beating yourself up for "not being strong enough" -don't forget how incredibly strong we are as survivors and how much we offer much of the time. We make awesome supporters! Don't forget all the times you have been patient, intuitive, understanding, gentle, and flexible. You're allowed to struggle. This is not a failure.
I was wondering actually whether your mother's comment could have activated some misplaced guilt for you. Something tells me there was an emotional barb in that comment somehow. But I don't know your mother, so I'm only guessing.
Let us know how therapy went, and if you guys have been able to communicate what is going on with each other.