You might look at the title and immediately jump to, "Wow, that must be a cuckoo crazy person." And you know what? Maybe you're right... but you'd be surprised to know that there are many a woman and man who has been in love and gone through the motions of being in a relationship with someone who never even knew that they were being sought after. Fact is, sometimes the other person is not aware of your existence, let alone that you have already formed a deep bond with them. For obvious reasons, this can be very unhealthy, and there are ways to break that hold:
Address it immediately. You've already crossed a certain point, so you might as well see if what you are feeling is mutual or unrequited. It is one of the most nerve wrecking things that one can do because not only does it leave you exposed, it takes your 'fantasy' and makes it reality. It's easy to keep playing relationships over and over in your head; in your mind, you control the outcome. But, when you reveal your feelings to the other party, the ball is in their park and it is no longer up to you as to how they will react. Only one of two things can happen: They'll feel the same way or they won't. Lingering on what-ifs is not fair to you or to them, especially if you let it affect the way you treat them in real life. Address it... for your sake.
Accept reality. There are those times when you would definitely need to address your feelings, and then there are those times when you simply need to take a damn hint. If the other person is in a relationship, or they have told you from the start that they are not interested in anything beyond a friendship, then it becomes one of those situations when you need to deal with that. I, personally, could not do that, and what that left me with was being a lonely spinster, hanging around like a scavenger, waiting for him to break up with some skank so that I could come in there and drop little hints. The worst was that I am gay and he was "questioning", but he would still go from girl to girl and I would just sit there hoping he would see he was gay... I seriously deluded myself. That tends to happen when you're in love with someone you cannot have and does not see you that way and never will... and you often have to accept it. After all, if something were going to happen, don't you think it would have happened already? And if you can't accept reality:
Remove yourself from the situation. This hurts the absolute most. It's like quitting cigarettes cold turkey; the withdrawal sucks, but the feeling of emptiness sucks, too. You've let this consume so much of you that it almost becomes woven into your identity. You wonder how you'll function without looking at his/her social media everyday to see what has changed. No more asking casual friends if they've heard from him/her or slyly inquiring about their location so that you can conveniently bump into them. This means taking everything that reminds you of him/her and getting rid of it. Why? Because most of the time, when you see them, hear them, or have that reliable go-to place in your mind, you'll never be able to move on and have an actual relationship with a human being.
Remember negative things about them. Can't think of any? Make some up. I mean, you might as well go there, because you're pining after someone who may not even know you exist, so they're walking past you, blissfully unaware that you're loving and hating them all at once. Remembering or thinking of negative things allows you to tie negative connotations to that person and thus you won't feel this incessant urge to curl up in bed and die whenever a romantic song comes on the radio, or if you see someone on TV who sounds and looks like him/her. It became easier for me when I thought about the fact that the guy I liked had a terrible personality and could not sustain any kind of lasting relationship. But you'd be surprised the things you'll overlook when your nose is open.
We've all been in love with someone who didn't love us back, but some of us have gone the extra route and have gotten emotionally attached to the point where we have to treat it like a real breakup. It may be out of the norm, but know that you are not alone and that many of us have had that feeling. You're not a crazy person, nor are you sad. You are just someone who loves hard and needs to channel that love into something healthy for yourself. Never waste your energy on hypotheticals; you're better than that.
Okay, I'm done.