You don't "owe" anyone a marriage. You do, however, owe it to her to be honest about your misgivings.
Never conduct a relationship under the hopes that your partner will "change". If someone has to change who they are fundamentally to be right for you, they're wrong for you. And I don't mean they leave their dirty clothes on the floor and you want them to put them in the hamper. I mean a change in temperament, personality, finances, etc.
If you have these misgivings, why do you continue to have her live with you? Is it your expectation that she should be willing to live together indefinitely without a marriage?