You should try to figure out what White is trying to do by "changing moves," as well as what White failed to do.
White failed to prevent you from moving into the center (and outflanking him) with a. That is something you should consider doing. It connects your upper side to the upper left corner stone, (and makes both absolutely safe). In a seven stone game,...
W1 looks like a small mistake to me, if any. This move indirectly protects the cut at the right side, so the third line stone cannot be cut off easily. It is, however, not strongly connected. White needs to be flexible, in case the naive result (the 3 stone group) is too dangerous.
W1 should be N11, then black dies, I'd claim from memory.
W1 at R13 is probably not a complete disaster in itself, but O12 should then be O11, or maybe P10 should be O10. That would force black to capture and white would at least build some influence -- not thickness, as she can't protect all cuts.
Even before that, white should probably descend at T13 and ...