Harry uses his trauma as occlumency

Pain flooded his body as Snape began yet another occlumency lesson. His worst memories repeatedly being drawn to the surface, and as much as he tried he was powerless to stop them. But this time he had a new strategy to try, focus on a bad memory and try to draw Snape in.

He endures memory after memory waiting on the one he wants to focus on, the time when Ripper chased him up a tree. THERE IT IS! FOCUS.

Harry focused on the memory making it more real and the emotions tied to it oozing off like condensation on a bottle. With a mental pull Snape was now the boy being chased up the tree. IT WORKED!