Dear Stephanie,
it sounds like your intuition sees this for what it is --- a rebound relationship. Don't be insulted, there's no comparing you to the previous partner -- but there is, I think, a lesson here. What is his ideal relationship? What will he be looking to replace now? What kind of qualities does he appreciate in a life partner? People will explain themselves to you all day long, but it is instructive to look at what they actually do. Do they look for partners to wake them up, challenge them, spark passion? Or do they want unconditional acceptance, accumulate various ex-lovers and ex-wives while drifting through life and demanding/expecting everyone to be friends....? Jackson Pollack's mistress wrote a book about their long-term affair which you might enjoy idly flipping through. It isn't exactly your situation, and she is not the most subtle judge of character; however, if you want a book-length exposition of my previous sentence, that would be one place to look.

That said, -- you don't need to berate yourself too much for what happened. It isn't unreasonable to have considered the possibility, but your intuition should not be ignored. As I'm sure you know, part of being an adult is being able to enjoy the satisfaction of a piece of cake without needing to finish the whole thing simply because it's there, or simply to see what will happen.

Mia made an interesting point about whether ideal relationships last and just to bring up the converse, I think relationships which are not ideal are almost never magically fixed. You might certainly decide to stay with him for some reason -- suppose, for instance, you found yourselves the only two people in some faraway country and his ability to remind you of home, to orient you, was worth putting up with a lot -- but don't kid yourself that it is because all shall be resolved.

Imogen's point about who is he looking to replace now? is a good one.