I have enjoyed dozens of platonic friendships with girls and ladies in my life, and would have enjoyed a romantic relationship with most of the people involved but ....
if the relationship has not naturally gone that way, it's because it's not meant to, and trying to make it into something it's not is to break it.
You can put it back together, but it's never really the same, it's a bell you cannot un-ring.
The trick in working platonic relationships with someone you are attracted to, is to never give them even the remotest hints that you feel that way about them.
In my experience, a close platonic relationship has its own rewards that a romantic relationship would probably not have - a closeness that you can't get any other way.
If you can't maintain that level of friendship, and your friend's other romantic relationships would upset you, you make a decision, to carry on and live with things as they are, or walk away, and only you can know how that is going to pan out.
In my (considerable) experience of these friendships, one golden rule holds true - if a woman is interested in you sexually, she will let you know, and if she isn't sending signals, it's because she doesn't think of you that way.
As a woman, you won't be falling into the trap that means you think your attraction is reciprocated because you'd like it to be, rather than because it is.
You will be wise enough to know that some romances are not meant to be - and act accordingly to preserve your friendship.