First of all, I can recognize that you are a good guy, one who doesn't have fear to take responsibilities and that whatever happens you are going to give her your support, as a friend, as a boyfriend, as a husband, as a real man.
But also you have to use your brains, how old are both of you?, do both of you work or at least does one of you have a job that is stable?, do both of you study?, what does she really want?, what do you really want(maybe you know it better than anyone in this world)?, does the new little life has your DNA and hers? In the last question, nowadays you have many options to prove it before the pregnancy ends.
Did you ask her if she wants to have the baby?, if she said yes, then you have to tell her that then she has to know that she has to stop thinking about only herself and put as first priority the needs of the newborn life. Ask her if she feels prepare to take on the responsibility even alone.
Remind her, without been impolite and aggressive, that you have responded to her and you haven't cheated on her since you met, so she has to do the same if she really loves you and if she wants to form with you a family. You did good to tell her that you love her and that you had many bad experiences in relationships by cheating, remind her that again.
Whatever is the result of the relationship, the baby's sake differs from it. If the baby has the DNA of both of you, then support him/her and love him/her, try to be a good father. If the baby results to be not yours, then you have to totally distant yourself from both of them, your girlfriend and the baby (unless you want to be his/her father). Your aim will be stay for your child, and only for your child, your girlfriend can support herself because she is physically and mentally stable, isn't she(no mean offense, but she isn't handicapped, and/or doesn't have any terminal nor mental disease, does she?)?