Ask yourself why you are only interested in men who are attached. Some psychologists claim that that is a way of avoiding commitment. Let me explain. If you meet a great guy and you fall in love then that is scary. But if you fall for someone committed then you can't have a long term relationship where you may get very hurt. This is just one theory. Some others are equally as scary. Are you punishing yourself. Are you suffering from low self esteem. I am just trying to look at the options.
You really do deserve a long term fulfilling relationship. One option is to see a counselor and maybe there is a deeper issue. One possibility is to find the type of guy you want in available places (church, internet, friends). Allow yourself permission to get to know them through casual dating with no pressure. You aren't going to like every guy you go out with so don't criticize yourself for that. To break the cycle you cannot allow yourself to fantasize, romanticize or anything else with any guy who is taken. Romanticizing increases the attraction - meaning focusing only his good qualities