Unfortunately, it sounds like you two have met at the wrong time in your life. First off please don't refer to her child as a mistake. I'm sure she would say it was a surprise but not a mistake. You need to be honest with her. It sounds like you love her but you feel you are too young to take on the responsibility of parenthood or step-parenthood. Which is totally fine and valid. However, if she is planning a future with you and you aren't on the same right now you need to let her know. It may hurt her feelings at first, but in the long run it's what is fair for all three of you.

I am almost twice your age with young children and it is hard and a big responsibility. I would not have made a good parent or step-parent at your age and I'm happy I didn't take that on until my thirties.

Good luck!