This is completely natural. It's basically human nature to chase novelty and the excitement of discovering a new person, having sex with a new person, and more. However, what you do with them is up to you. You can suppress them and choose to continue with a relationship you're sure about with a happy future, or you can chase the unknown. 

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Now, this desire for novelty comes partly from the Coolidge Effect, and partly because of the drugs your brain releases when a new potential mate comes into the picture. I shouldn't have to say this, but the wise choice is to not be a hormone-controlled teenager and just suppress this feeling. Cut this new guy out of the picture completely, because we both know that you're current indecisiveness has created a situation where you haven't removed this threat but kept it on the sidelines in case you wanted to go and get your brains fucked out - And like I said, it's just biology which is dictating this. No shame in it. Acknowledge that these feelings will exist, and then deal with them accordingly, to ensure your relationship remains safe.