Girlfriend thinks we are falling apart

Talk to her and ask her why she thinks that and how you can change it into a better relationship. If she wants changes, stick with your word and maybe it will get better but you have to stick with it. If you don't, then she won't trust you and that's a big part of a relationship. So just talk to her and ask her how you can make the relationship better and what you think is going wrong in it and see what she says.

Reassure her that you're not...Tell her how YOU know you're not (the reasons). If she keeps saying it over and over, maybe she's the one falling out of the relationship.

Hey ask her to talk about it. It will show her that you're interested in her feelings and that you're committed to her. It's a rather obvious response, but what else can you do?