I can’t even start to imagine how hard this is for you. 

You're probably thinking to yourself:
"Why do I really need to break up with her? If we go so good together, why can't I just continue? You might argue that it's bad for my yiddishkeit, but I truly feel that I've really grown since I've met her."

Let me tell you, I once went to visit my family, who had a girl about my age. We started talking. I fell in love with her. More precisely, I was totally infatuated by her. I already saw myself together with her. 

Even though my situation wasn't so involved as yours, my point is that I wasn't seeing clearly. Even after not having spoken to her and seeing her for a few weeks and months, I still felt a 'connection'.

I find the problem with these situations is that as long as we're involved, and perhaps as long as we see it as a possibility we aren't able to think clearly.

Therefore, I would highly recommend what has been recommended by the choshuve oilem over here: either seek daas toireh on this issue by someone who knows you and find clarity, and if he tells you to marry her, then fine. Or break it off. 

The situation as is can't continue.

I know it's an extremely hard situation on your part. And the fact that you were able to reach out over here, makes you a true hero. But the fact that you reached out over here shows that you yourself realize that this situation is not optimal. It's probably tearing you up on the inside. That's normal. 

You're at a crossroads where every option is painful. 
1.) Reaching out is extremely hard, especially with the nature of this issue being very sensitive.
2.) Breaking up doesn't even seem possible. How can you get yourself to break up with your best friend, especially if she might be your bashert?
3.) Continuing the way it's gone so far might be the easiest option and that's why it has continued up till now. But you yourself realize that it's not right and it's eating you up on the inside.

At the end of the day, Hashem created each one of us with a moral compass. You know what the right thing is to do, and as long as you won’t follow that inner guide, you’ll feel that bitter aftertaste.

May Hashem give you the inner strength to do what is right in his eyes.

AM