(Firts of all, sorry if my English is not perfect but it's not my first language) This is a bit complicated: I've always been friend with a girl (let's say that her name is A, since I don't want to say her actual name) since middle school. She's always sad because of lots of family problems but I always helped her and also, I'm pretty much her only friend. About one year ago I finally told her that I'm pansexual and she was totally okay with it, but the problem is that, a few months ago, she started hinting me that she had a crush on me. Then, she directly told me that she loves me. I asked her about her sexuality because actually I thought she was straight, and she told me that 'I'm not sure about it, but I only know that I love you'. Unfortunately I see A only as a friend, but I didn't want to let her down, (she was already depressed because of personal problems) so I said that, if she was okay with it, we could try to hang out and see what happens (I thought that maybe I could change my mind): she agreed. Now, it's been about three months but nothing happened- actually nothing good happened as we had many problems trying to hide our relationship since none of us is out of the closet, especially to our families. I realized that I see A only as a friend and time can't change it. I know that I was supposed to tell her everything since the beginning, but now it's too late. So how can I tell her without making her even more sad that she is now?
Unfortunately if she really does love you, she will get hurt but try to be as kind as you can in a way you deliver the news. Tell her what you told us, that you probably should have told her from the start that she was only a friend. She might understand with a little time.
It will probably only get harder the longer you wait, but when you tell her, don't forget to stress that she's your friend and you care about her. Also, you don't have to tell her everything. You can tell her you went along and tried out having a different relationship, but found you're still only into her as a friend. It's all true. The only thing I'd suggest leaving out is that you were certain from the start that it wouldn't be right for you. That would hurt her even more, and don't most of us prefer white lies to painful, pointless truths?