I'm going to go to a different girl
What is the reason why you feel yourself interested in another girl now?
A one night stand is all I need, I don't need another woman in my life.
It's not right to bounce around between relationships too often, but if you are no longer interested in your current relationship, it's fine to leave it.  Just make sure that the woman you are going after is also just looking for a one-night stand, and doesn't think she is entering into something serious.
I'm not sure how to do this. I'm good at meeting one night stands, but I never know how to be rejected.
You can not have a one night stand if you are in a relationship with someone else. You need to end your relationship if you want to have a one night stand, because cheating on your partner is a terrible thing to do. Finding someone new is not a terrible thing, but is instead the start of a new chapter in your life.