how to break things off with a girl I'm dating?

We went on 3 dates, she really likes me, I think she's pretty but kinda boring and not very smart. It doesn't help that I met another girl I've been on 2 dates with and I really like her. The girl I wanna break things off with is kinda clingy/romantic, she keeps talking about how much she likes me and all the things she wants to do together and last date she kinda opened up to me talking about her rough childhood (dad passed away early, mom's an alcoholic, raised by granddad). Now I wanna break things off but I'm not sure how to do so without hurting her more than strictly necessary.