Most Helpful Girl
Don't bring up breaking up. Don't even ask her if she really wants to be with you. She's with you right now, so focus on keeping it that way.
Rather than questioning her motives, give her more reason to calm any doubts you may have and be confident that she IS happy to be with you. Take her out to her favourite restaurant, offer to give her a massage if she's stressed and throw on a few random kisses on her neck/back if she agrees to the massage.
Most Helpful Guy
What's she doing to give you the impression she's wanting to move on without you?
Sometimes the only way to find that out is to openly address the issue. Sure, you don't want to come across as insecure and/or needy, but when you're comfortable with someone, you can generally get vibes when their feelings change.
You can look back to see if there's anything you did or said that could have turned her off, or, most likely, can you remember about the time she started behaving differently around you?
You don't even have to ask her if she wants to break up with you; rather ask her if anything is wrong, because you've noticed she hasn't seemed her usual self. If she gets defensive or doesn't want to talk about it, then you might have reason to worry. If she comes out and tells you what's bothering her, and it's not related to you, then maybe you can help her deal with it. On the flip side, she may come out and say she wants to date other people, which isn't good, but at least you won't be worrying yourself sick over what she's thinking.