I feel for you. I am some what on the same boat.
I had a bad pregnancy with my daughter, then she was in intensive care for a while.
I have never felt the bond with her, I breastfed her and co sleep but I feel my love for her is so little comparing to my love for DS. DD makes it really hard for me to love her.
Hubby on the other hand not only treats her like princess but spoils her greatly too and he is extremely harsh on DS and we always end up in fights about it.
I don't know what to do but to try harder.
It's such a sensitive topic and takes a lot out of someone to admit.
Perhaps try talking to someone, talking to hubby, or GP if it's making you really depressed
Sorry boy much good advice, I am a bit lost myself.