What you can do is back off being sexual and focus instead of being friends and getting more comfortable with each other. You’ve only known each other for a month. That’s very soon to be asking a woman who has been abused to be intimate. I know that at 19, this may seem a lot to ask of you. But if you really care about her, spend more time talking and doing things together and bring the messing around down several notches. The intimacy will evolve as the relationship does.
To your girlfriend: I hope you will consider getting some professional help. The consequences of abuse by a family member can be long and deep. But you don’t have to continue being so afraid. A therapist can help you put your past where it belongs. Meanwhile, it’s more than okay for you to say a clear “no” to intimacy you’re not ready for. If this man loves you, he’ll love getting to know you and will be happy with some hugs and kisses as signs of your interest and affection.
I wish you both well.