"I'm single, but you're mine." If you are in a "relationship" like this, first of all, it is not a relationship and second, you need to get out. I've seen this happen too many times to friends of mine and I have also been in "situation-ships" such as this. It's not healthy and it is not fair to you. Is this fun for you? Do you enjoy this? Although the person stringing you along might not have come right out and said this but I think deep down you know the truth. Whether or not you chose to put on your blinders and see what's happening or not.
Maybe you don't want to end it right now because you want somebody around, you need somebody to give you that attention. Perhaps... but that is directly disrespecting yourself. If you don't allow other people to disrespect you, how can you allow yourself to? You need to love yourself first and foremost. No one should have you feeling committed without the commitment. If they aren't willing to work this out or they continue to treat you like this, it is your choice to get up and walk away. You can have everyone you trust give you advice and tell you the cold hard truth, but in the end, you make your own choices. You decide with what you want to put up with and if you want to be respected or not.
I know it doesn't feel good to be in something like this. You can't have a half relationship. I know you feel like you can't get out- like you don't have another choice, but I'm telling you that you do. I can only hope that you will see what you deserve and realize that you need to love yourself. Love yourself, set boundaries for yourself, and respect yourself. I am not saying it's easy to just get up and leave, because the truth is, it's not. You're going to want to go back, they are going to try to pull you back. You know that you might go back and fall into that situation like a safety net, but hopefully you'll start to open your eyes to the fact that love blinds you. I am a firm believer in that.