She says we are not compatible!

My girlfriend and I have been together for six months and she keeps telling me that she thinks we aren’t compatible. I don’t know what she means because when we’re together, we have a great time and manage to talk about a lot of things. When we aren’t on a date, and are just chatting online, she talks about how she isn’t sure if this is going to last because we have little in common. When I ask her to specify, she is always vague, and it feels as if she changes her mind when I’m not around. I don’t know how to bring some element of stability to this relationship because if she keeps going back and forth about this, I will lose conviction too. Is there anything I can do to get her to be sure of herself or what she thinks of me?
It may be that she needs time to understand what she feels, not just about you but from what she expects this relationship to be like. Why not suggest she take some time out and think about what she wants? It may seem like a harsh idea, but this isn’t a suggestion that you end the relationship. You can explain where this is coming from, why her inability to commit to something is leading to instability, and how you want this to change for the better. If she can think about this deeply and come back with something specific, it can help you both. Ask if she wants this relationship to work because that is the only thing that matters. If she does, everything else can be resolved over time.

How does one get into a long-term relationship? Everyone I date tends to move on after a few months and no one has stayed for even a whole year.
Be honest with yourself as well as the person you’re with and talk about what you want to work towards. Also try and be more circumspect when you get into a new relationship.