You know what strikes me about that paragraph lifeissweet has quoted, is that's it's all about what she does for him.
Does OP listen to her, support her, go out of his way to do lovely things for her, wish her to have the most wonderful life possible? Is her happiness her top priority, after his child?
She sounds like an objectively perfect girlfriend. A really well-adjusted, funny, kind, interesting, thoughtful, all round lovely person.
Almost as if she's making everything too easy for OP. 'All' he had to do was get together with her and this perfect relationship, offering all the normally desirable opportunities for progression, was presented to him on a plate.
I wonder if it feels too much too soon, in the sense that there has been little challenge or negotiation required, to reach this 'perfect' point?
And/or, whether she is too perfect, too mature in terms of stage of life and approach to relationships, for him, at the moment?
Maybe you need to kiss a few more frogs OP, and/or work through a few feelings of your own, before you work out who you really are, so what you can offer anyone, what you really want and who you really want?
Maybe she'd be better off with someone who already knows themselves better and is ready for a serious, settling down together relationship?
My guess, based on the little you've given us, is that you feel that she probably isn't 'the one' for you because you know, deep down, that you are not the right one, for her.