Why do you get involved with someone exclusively if you aren't ready for a relationship? Maybe you should consider casually dating different people until you are ready to be committed to one person (if that's what you want).
For me, finding a balance between maintaining my independence and putting effort into a relationship is really difficult for me. I need some time to myself, to be able to do things that I want to do - alone. I don't want to always have to think about another person's feelings, or what they are doing, all the time. Maybe you are finding that you are spending too much time with your girlfriends, and you need more time to yourself/time with friends/doing activities without your partner. It's OK to do things without your partner and still enjoy them.
As for not being able to initiate a break up... I think a lot of people find difficulty in that. It's not easy to have that kind of conversation with someone you care about. It's a vulnerable thing to express your true feelings and to be that honest with someone. Also, not being able to predict how the other person will react to the conversation might add some anxiety. I think the best way to go about it is to be up front and honest, especially if you've been thinking about it for weeks. Bring it up in a calm way, and use gentle language and tone when you talk to her. Maybe say something like, "I've enjoyed our time together, I think you are a great person, but I've realized I am just not ready for a relationship right now and I think it's best if we go our separate ways." That seems the easiest, most diplomatic way to say it, and most people will react to that calmly. Sure, they may be hurt or confused, but you can navigate the conversation from there, and try your best to be considerate and compassionate to them while you are having the discussion.
"Caring is not an advantage."