Advice about a crush, please...

There's a kid I like.
But I don't think he likes me that much.
He recently got a girlfriend.
He asked me to do something special for his girlfriend's birthday so I said I would.
At least, that's how I answered...
I don't know why I feel this way.
The fact that he has a girlfriend is none of my business and I have no right to make it my business...
Even if I don't want to see him for a while, I have to be with him all the time and it's stressing me out. What should I do?
What should I do when I feel like this?