Can you believe that I'm in love with my wife's best friend
That's not good. How did you fall in love with her best friend?
She was around a lot so I got to know her, and she makes me happier than I've ever been.
It does feel good to find somebody who makes you happy but you shouldn't be in love with somebody else when you are married. It will hurt your wife and make people think less of you.
I don't think we can just be friends and leave it at that.
If that's the case you need to tell your wife. You shouldn't continue seeing the best friend while your still married to your wife. You need to make a choice, and give your wife the option to move on too.