What do you think of me being so excited about winning that I stopped playing very well?
That sucks. What made you stop playing very well?
I was so excited to win. I got over-excited to beat him. I wish I hadn't played so well!
Winning does feel good but it is okay to lose if you got too excited to pay attention anymore. It happens when people are really happy that things are going well.
Oh well. You win some, you will lose some, who cares.
Yeah it already happened so don't stress. But don't blame  yourself too much, it is normal that you got really excited about winning that you got careless.