Girls, I have a girlfriend but can't stop thinking about someone else?

Anonymous
Please don't resort to name calling (jerk, bad boyfriend, etc.) I'm looking for some insight here.

I'm dating a girl I've known for a long time and I feel like I'm a good boyfriend, I treat her right, etc. But there's another girl I can't stop thinking about. I'm totally crazy about her, I think about her all the time, especially on weekends, re-read our text messages, I get turned on just by thinking about her. But I know it isn't really fair for my girlfriend and I'd hate to leave her because of this, that's wrong.

Any advice? thanks.

Updates
+1 y
Tons of great advice, thanks ladies.
Girls, I have a girlfriend but can't stop thinking about someone else?
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