lol wtf
I can't understand anyone that can claim to be 'in love' or have strong feelings for someone they've never been romantically involved with. it's simply impossible. It's one thing to have romantic interest in someone (wanting to see if you could be more than friends) or feel curious but claiming to have strong feelings like that is weird imho. The fact of the matter is unless you've dated someone you wont know what its like to be with that person romantically and therefore cant develop true feelings until then. It seems like you're just infatuated with the idea of being with her and you're being obsessive.
It also indicates you're not a real friend, and i suspect she'd feel a little creeped out if she found out someone she thought was her friend was claiming to view her that way for so long.
Personally if you have a genuine romantic interest (not to be confused with having feelings for someone) and you think it might be mutual and there's a potential for feelings to develop between you two then I dont think there's anything wrong with just being open and asking her out, but you have to not be immature or weird about it or turn it into some big drama. If she's not interested in you that way then it's no big deal, continue as friends and you'll both be happy. but thats very different from randomly telling your friend you're in love with them or have feelings for them for a long time. it'll just creep your friend out.
if its true that you feel that way then you obviously see her as more than a friend and you should be honest about it and end the friendship so that your friend doesnt get creeped out and see you as deceptive when she finds out.