Be Honest With Her
Call her up and tell her you have met someone else and you cannot continue your relationship with her because you have feelings for someone else. She may be upset at first, but she will eventually be thankful for your honesty. If she gets really upset, you have solace in knowing that the fling is over and you can now focus on building a relationship with someone else.
If you feel this conversation is better done in person, ask her out to a bar or café where there will be other people around. A public place will be a good option if she does get extremely upset with you. Just tell her that your relationship was never going to be anything more and you understand if she is upset, but you need this relationship to end.
Slowly Cut Ties With Her
If your relationship is truly casual then you probably see each other every now and then. It should be fairly easy for you to slowly cut ties with her over the next few weeks, especially if your relationship with the other girl has started to develop. If the casual fling starts to get suspicious of your absence in her life and bed, be honest with her and tell her you’ve met someone else and that you are not interested in having a casual relationship anymore.
Introduce Her To Someone Else
You have a friend who is lonely, or the one who has met your fling and thinks that she’s hot. You think the two of them would be great for one another. So, hook them up. Now she’s busy with someone else and you too can focus your attention elsewhere. If she starts to feel guilty about leaving you behind in any way, tell her that you too have someone new and wish her well in her new relationship. Though, this can be a bit tricky, if your friend is someone whom you see often and the relationship with your former fling doesn’t go well. He could be mad at you or things could be awkward because of your past relationship with that girl.
Breaking up with a fling can be hard to do, especially if she has feelings for you that you don’t have for her. But if you have found someone you want to be with and your fling is in the way, be honest with her and tell her you want to try something new with someone new.