Lots of people have been there, you definitely aren't alone in this one, and I know it's a horrible place to be. I was really good friends with a girl for a couple of years before I realised I liked her more than a friend. I started to think about her ALL the time, everything reminded me of her, and we were very close. It became so painful to watch her flirt with other guys (of which there were many as she socialised mainly with guys), as well as the pain of liking her in such a way when we were only "friends". I delayed asking her out as I didn't want to risk the friendship or make it awkward in our friendship group.
But obviously in a situation like this there can be consequences that you also need to consider. In my case I had to know if she might like me back, so I asked her out - I genuinely thought she would say yes after how things were going, but I was rejected. At least after that I KNEW it was unrequited love, and this gave me a green light to at least try and move on with my life. She wanted to remain friends because we were so close (we knew a lot of secrets etc similar to OP) and she made an effort in that respect but I just couldn't stay friends with her, as painful as at was, especially after she got herself a boyfriend shortly afterward. I felt that keeping contact would only remind me of what I could never have had, because deep down I still like(d) her. So yeah, this could also happen to you, but it's a risk you gotta take for your own sanity! Good luck