You love the girl you are with but after a few days another girl came into your life and you fell in love with her so how does a person know which girl is the one for you?

It's simple, you are immature and don't know what love is really about. You just don't trade-off girlfriends and end up hurting their feelings. I think you'd better take time out until you mature more.

Love is something that is really understanding each other