One way or another you have to tell her you're not ready for that. Sal Paradise had a pretty good response. You can also talk about past mistakes you've made, giving in to relationship pressure, which ended up hurting girls feelings, and you want to avoid doing that again.
Do not use the frame of "I don't like you enough."
Use the frame of "I feel you pressuring me and the right thing to do is to resist the pressure in order to protect you." If all goes well, she will respect you for being solid in yourself, and she'll appreciate you looking out for her, and she'll try not to pressure you any more.
