It doesn't seem right that your girlfriend can't also be your friend. He understands that she needs someone to talk to about him, but she should be able to talk to him about other things. She seems to, but he wonders how much she no longer shares.
He remembers when he first came to Everwood, before Colin came back and everything fell apart, when he was her friend and confidant. But they haven't been like that in a while, and he doesn't think they ever can be again. They managed to patch up their relationship after Colin, and again after Tommy and Madison and the mess that was the last year or so, but it's very different now. It's okay, though, because he loves her, and he's happy with things the way they are.
But he's always been one to overthink things, and so he's thinking about what will happen if one day they decide that they're better not together. Too much has changed between them for them to ever go back to the way things were. So even if he wasn't so much in love, he wouldn't let her go because he doesn't think there's any other way she can be in his life now, and he simply can't imagine life without her.
Is that love? It doesn't sound particularly romantic. He doesn't think that's how it's supposed to be, although he's hardly an expert. But, in the end, it doesn't matter because he does love her, and he'll hold onto her as long as he can.