You're just going to have to tell her how you feel and be honest. Whether it's in person or through text or whatever, you need to come out and say that you have feelings for her and you're not sure if you can continue to be friends in this capacity. Then you need to get some space. This is very important. You need to take time to yourself. If you can get over the fact that this girl does not want to be with you romantically then you can come back at a later date and attempt to salvage your friendship, and if you can't get over that fact then you need to cut her out entirely. I understand that this would be very difficult for you to do, but you are not doing yourself any favors by continuing to talk to her with these unresolved emotions going through you.

A couple years ago I had a similar situation with my closest female friend. We had built a bigger trust and understanding over the course of a few months after being more casual friends for a few years. It got to a point where I told her I wanted to pursue a relationship with her but she didn't reciprocate. We both understood the risk I was taking to our friendship just by me putting it out there. Things were awkward for awhile and I was down, I thought I had lost what could have been a great relationship and what already was a great friendship. But I moved on and refocused my energies elsewhere. She's now in a loving relationship and I've started dating someone recently and it's going pretty fantastic. But the funny thing is at this point we are probably closer than ever, but I'm without the lingering feelings that were dominating my thoughts before. All in all I don't think I could be happier with how things turned out.

My point is that this won't be the end, you may still end up with a great friend when it's all said and done. But that is entirely your decision. If you can handle her being a friend to you and nothing more, that's fine. If you are unable to move past your feelings for her then that is fine too. Just don't put or keep yourself in a scenario that eats at you from the inside. Doing so is what perpetuates all the stupid "friendzone" shit and you're better than that. Find your self-respect and do you first. You got this.