It is better for your little one to have happy parents. And if she is little it MAY be better to do this sooner rather than later.

I suppose the best thing to do is actually tell him you are thinking of leaving, that you feel you are both unhappy and no one should live an unhappy life. No one has to be to blame.
It would be good to give him to to think about it. Some guys really have no idea that we are unhappy. He might want to fight for the relationship, might not but at least you will know.

I am happier single, I just am. There is also something very satisfying about it - captain of my own ship yadda yadda yadda...

ETA - yeah, but I don't think humans are supposed to live without affection, its a big thing. Oh and him avoiding the whole subject sucks a bit, its cowardly.