I have no problem being hard on myself.
Sadly, that critical ear, for me, has always come with more distance from the creation process. I record a song and then, a year or so later, I start hearing all its flaws. I think that's a patience issue, though. I record and just want to GET IT OUT!
Currently recording a couple of new tunes and, I think I am getting better at the patience thing. We'll see.
As far as critiquing the song, itself: I think that I am simultaneously my own harshest critic and biggest fan. There are songs that I won't record or release that other folks seem to like a lot -- I just can't connect with them. But then, there are others that I love, that seem to get a really lukewarm response from the public.
So yeah, it is hard for me to be objective, I guess.
It's such an internal, emotional thing.