Falling in Love with a Best Friend??

This is not exactly a love story, and I'm not sure if it should or could turn out to be (skip to the last paragraph or two if you don't feel like reading.)

I'm a senior in high school and I have been close with my best friend since sophomore year. Before we were friends, he had a crush on me but his friend at the time (who also wanted to date me and did for two months) discouraged him from saying anything, playing on his insecurities.

I actually didn't know that story until junior year. By that time, a lot of our friends noticed the way we interacted and suggested we date.

We didn't listen we've always been attracted to each other so we came very close to being friends with benefits, I decided I had feelings for him, and I went a little crazy. I essentially tried to keep him single by not letting my girlfriends go out with him and telling him problems with a few of the girls he wanted to go with. Turns out, I was always right about those issues, but I still feel bad about my intentions in telling him these flaws.

My feelings for him got so strong at the end of junior year that it was very frustrating for me. I wasn't communicating very well, we had a lot of fights, and for months, our friendship was over.

But for some reason, we started speaking again and picked back up. It was so natural for the both of us, like our friendship was meant to be. We tell each other everything, hang out regularly, flirt a little, etc. We each have our own set of friends, so our relationship tends to evolve off campus although everyone knows that we're close.

tl;dr: Now my best friend and I are both open to flings, but seeking long term relationships. My female best friend of 14 years has seen us together and thinks we should date. I know that he wants to hook up with me, but I've rejected him out of fear for our friendship (he knows why). I love him so much it feels impossible to explain, but I'm not sure if it's a romantic love or a platonic love (or if it matters.)

Is it healthy to hook up with the person you're closest with? Is it normal to at least try a romantic relationship with them, and how do you go about that "trial" without messing anything up? Or should I just adhere to that old saying, "If it ain't broke, don't fix it?"