if you really like the girl youre in a relationship with , you wouldnt even think or let the new girl approach u at the first place. its your choice to think if your current relationship is the best for you 2 and talk about it, honestly. Not sure what your age is but both of you still have plenty of time to focus on yourself and to find someone even more suitable. if you dont wanna break up with the current gf, stop talking to the new girl , cut her out from your life, you dont have to be rude about it but you need to be clear to her the boundaries between you two.