Does anyone else have trouble dumping people?
Are you just scared? Realize that you are actually just placing the break-up into another (future) period of your life, in which it may actually be tougher to deal with.
Are you afraid of hurting her? Realize that it is actually what's best for her (even if she doesn't agree).
Look, despite what people may think is "fair", you don't have to give any reasons for dumping her. You don't have to point out any flaws or any of that crap. Just tell her that right now you would value the freedom of being single over the comfort of being with someone. I've learned the easiest breakup is one that doesn't point fingers or leave anyone with a feeling of guilt. You are a human and have every right to choose your own path.