Unsatisfying relationships are depressing. Also, you're probably upset with yourself, perhaps in an unacknowledged way, for spending time with someone you know is wrong for you. Sure, you tell yourself that you're only going out with Slimey Sam or Nutsy Nancy until the right person comes along. The problem is that when you're depressed, you're not at your best and most attractive, so you're not attracting the right kind of people. Worse, you may be actively repelling the right kind of people.
People hear about other people's reputations.
If Mr. or Ms. Right is attracted to you but sees you out with someone who's known to be a sleazeball or an airhead or a loser, he or she is going to conclude that that's the kind of person you're always attracted to. The only answer is to bite the bullet. Get Mr. or Ms. Wrong out of your life, so you can be happy and attract a really nice person. It's better to fight being a little lonely than to miss the right person when he or she comes along. Cold turkey is the best way to get out of a relationship.