I met this girl awhile ago and things went really well, and we got into a relationship but lately I haven't been able to keep my focus just on her. I have found myself flirting with other girls a lot and I don't know how to stop.

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Do you believe in miracles!??!?! YES!

Flirting with other girls isn't the worst thing in the world. It keeps what made you attractive to your girlfriend active and in-shape. It sounds like you think you're overdoing it, though.

One of two things crosses my mind:
One, you're not ready to settle down yet. There's nothing wrong with that. If that's the case, be honest about it with yourself and with your girlfriend. It will hurt both of you less.

Two, you are suffering from an addiction. Didn't David Duchovny check into a rehab clinic for sex addiction? Pickup addiction could be the same way. Try rational / emotive / behavioural therapy. Acknowledge your problem in writing and aloud. Possibly even admit to your girlfriend that you've having a problem. Make a habit of resolving daily (aloud and/or in writing) that you are going to be faithful to your girlfriend to the satisfaction of your own conscience; and focus on how good it makes you feel to be faithful to her, while you do it.

It might sound ridiculous, but just like getting psyched up before going out sarging gets you into a flirtatious mindset, this establishes a pattern in your brain of determined, deliberate, and gratifying faithfulness.

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Some vices miss what is right because they are deficient, others because they are excessive, in feelings or in actions, while virtue finds and chooses the mean.
Aristotle, Ethica Nichomachea