I'VE BEEN BEST FRIENDS WITH HER SINCE 8TH GRADE HOLY stuff AND I DONT WANT TO RUIN A FRIENDSHIP THATS GONE THAT LONG
That won't ruin it. If she says no, you man up, say "okay, I understand, thank you for being honest" and move on.
you may think you'll wanna T-bone her like a semi-truck into a little suburban on a downhill slope when the brakes give out even though she said "nah brother," but you'll move on. Don't gimme that "it's fate that we fuk do adin" bullstuff. You're mentally wired to move on if she turns you down.
That, and if she stops being friends with you for something like asking her out, she's not a friend. It's not like you said "damn bby I'd take dat ass like I take my liquor; HARD."
What if 10 years from now you and her are talking, and she says to you "you know when we first met I had a massive crush on you," and she says that as she holds up a picture of her fiance. Sound fun? No? Ask her out. You literally have nothing to lose.