Table tennis question?

I have been regularly versing my best friend at table tennis for the past 4 years. He has been playing tennis since he was six so he was far better than me to begin with. He never went easy on me, so I would always get throttled until I developed to his standard. Because he constantly bombarded me with power shots, I quickly developed a strong defensive game, and can hit back nearly anything. The problem is, I have absolutely no offense and cannot hit winners. I rely primarily on his mistakes to win points. He is the only person I ever play so how can I improve the offensive side of my game?