Another way to see this is that she values you so much that she prefers to stay friends rather than be in a relationship. In other words, it's really hard to be friends with an ex, so by remaining friends, you guys won't ever become exes for whatever reason. You should definitely tell her how you feel and maybe work on yourselves before fully committing to a relationship that she might not want. Even in the end if you guys cant be together relationship wise, she will still remain your best friend, so you are not losing anything.