Dr. John Watson thought he understood what it meant to be broken, though for much of his life he'd had no personal experience of that state. It had always applied to someone else.
For example, that his sister, Harry, needed an alcohol-induced haze in order to face the world was a clear indication to him that something within her was broken, but if she'd just be sensible and seek treatment, he was sure it could be fixed.
His tour of duty in Afghanistan gave him a whole new perspective. The wound to his shoulder stripped him of his sense of self: His career as a soldier was gone and his career as a surgeon in question. He came home to England broken and discarded, a man without place or purpose.
Sherlock Holmes had changed that, had rehabilitated him, both mentally and physically, better than any medical colleague could have done.
Then, driven by shame, Sherlock committed suicide. He'd held his hand out toward Watson and then stepped into space. Watson would never forget the sight of the fall, the sound of the impact or the feel of the pulseless wrist. PTSD was a problem with forgetting, not with memory. As he stood before Sherlock's grave asking for one final miracle, he knew that this time something inside himself was irreparably broken.