I think I'm falling for one of my best friends, what do I do?

I (M20) have been friends with "Nadine" (F20) for about 6 years and at first I genuinely viewed her as just a friend. As time went by we became so much closer and there has always been a thought at the back of my mind that we can maybe become something more than friends but I never entertained the thought cos it would be weird.

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Fast forward to last week, she comes over to my place (she does come often) and we just spent most of the hangout playing video games and just chatting. I really enjoyed her company and it was this moment that I had developed a crush on my friend.

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I guess my question is how do I approach this with her without ruining our friendship?