I can't get over how Mycroft's life was pretty much circling around being dragged into a web of lies and tasked to uphold it to protect those around him, and then it turned out they didn't want to be protected. And he got to hear about how stupid he was. But they did not reject him, because they did not reject their daughter either, no matter what she had done.
I think he carried around that fear of rejection all the time before this point. He tried so hard all the time. To seem grown up. To seem cool and dangerous. To control Sherlock, only to have him perpetually push him away because he was so intrusive.