How can I tell the difference between not liking a girl, and not wanting to like her?

I've been dating a couple of girls recently, but I quickly lose interest in them when I start to get to know them better. For a person who's a little bit afraid to get too involved in romance, how can I tell if this girl that I'm dating just isn't the one for me, or if my emotional defense systems are kicking into overdrive and being a bit overprotective?