Did you ever got Friendzoned? Did you ever dated a girl for a while, maybe you kissed, maybe you even had sex once or twice but soon after she wanted you to just be friends? that she likes your company but not in a romantic way?
well, to me it happend, even twice :)

Im a proud owner of two friendzoned friendship with two women I originally knew on tinder..
It is always a bad feeling at first. Its a really hard blow to your ego to get this talk from a woman you've been dating and you can easily get angry at her for asking this and refuse, but I didnt. The reason why is because I did found a common ground with those women. a different kind for each of them and its not so easy to find this with the opposite sex.

most women are complicated and they always try to find reasons not be serious with someone and not have sex. They have to find that special guy who will cause them "butterflies" in the stomach and then they will "know".
I believe that this "butterflies" feeling is something that is needed to buildup inside a woman and it can only happens within the limits of friendship.  only when she remove all of her natural defense mechanism from you when you show intent of being romantic with her which can happen if you became her friend for a while, only after you became comfortable with each other doing anything together without any pressure to fuck it up, this is your chance, if you want it ofcourse, to make a move again and show intent.

you can tell her that you cant stop thinking about her, you can tell her that you will have to end this friendship if it will not become something else, you can do it in every way you please but I assure you, that woman will think really hard what is best for her before she turns you down as opposed to her turning guys down that only wanted to get romantic with her.  She will consider your good friendship and your common ground in the time that you've spent togheter , and she will appreciate it and recognize that this is a hard thing to find . 
I really think that the best romantic relationships has grown from great friendships.