Why do I keep on liking guys when they clearly have no interest in me?

I am really confused. I have a miserable life. I don't have any social ties and barely manage to survive each day in my life because of all the depression, anxiety and stress that I go through. And one would imagine that a person like me would be the last person in the world to be experiencing such things as attraction for the opposite sex and the like. The thing is once in a blue moon I always end up extremely liking a guy and fantasising of all the things that could happen between us which won't. I know I am not lovable but there are just moments when I'm so consumed in emotions I forget my place in society. My last infatuation with a guy ended very badly. The reality of the guy never ever liking me back hit me so hard it took me years to recover from it. And now I feel as if I'm falling back into old habits. I'm starting to like this guy and I don't want to. I don't want to lose focus of the things that I need to be on top of. The last thing I need is a destruction. Please tell me how to just switch my emotions and stop liking this person.