My girlfriend is getting more and more pessimistic, saying that she shouldn't have children.

I think that when two people are together, there must be a reflection on the future. You should tell her clearly when she is depressed that her grades are not good. Then you will raise her and let her know that you have never been disappointed in her. When she falls, tell her: I really don't know what my life would be like without you. So she will feel that she is still valuable. Then study with her and help her catch up with the progress. It must be hard to study, but you can't be soft-hearted. You should force her to study hard. You're gonna be okay. Love well and don't give up easily.