At the end of 1984, what "victory over himself" does Winston realize he has won?

The line you refer to is at the end of the book in Part 3, ch. 6.  Winston has gone through the brain-washing and torture.  He has confessed and he has implicated everyone he possibly could implicate.  When a memory from his childhood enters his head, he pushes it out as a false memory.  When he looks at the huge portrait of Big Brother hanging in the café where he sits drinking gin, he understands that he loves Big Brother and that he has no doubts about his loyalty to Big Brother and the Party. This is the "victory over himself".  The brain-washing transformation from a person capable of independent thought and independent will to a person able only to think in automatic, controlled thoughts is complete.  Winston does not think for himself any longer.  He thinks the way the party wants him to think.  He cannot reason any longer and he cannot remember anything different from what the Party tells him.  Since he has lost the ability to think for himself, he is no longer a person, so, in essence, there is no longer any "himself" - he has been defeated completely, thus giving the "new" Winston a victory. The Party will now kill Winston and he knows it, but he loves Big Brother all the more.