The 'spark' that you are referring to @devilish does not disappear in a healthy, working relationship, just because some time has passed. It evolves. There is a stark difference. The spark of the initial stage of a romantic relationship becomes deeper in an ideal world, the pink clouds become something real, you see the person for who they are, and love them as they are - again, in an ideal world.
You keep being on an off, something is clearly dysfunctional, it is not cute or romantic that you find your way back to each other, it just means that you revert back for comfort. You know for a fact something is missing in you, there is no question about it, so not much to do there. Just because he is a nice bloke and in theory it should work, doesn't mean it does, humans don't work that way. And if you are considering settling for the comfort, that is a horrible step, given that you have a daughter. Kids are not dumb, they grow up absorbing things being normalised around them.. your kid will grow up with mum being in a relationship where she clearly is not in love, only settling for comfort.
Be honest, would you like seeing your daughter in the future settling for a relationship like that? I am willing to chance that you wouldn't.
I always say it's better to be alone than with the wrong person.