I feel for you as I have a little bit of a similar problem - but different
!!
I have a 6 yr old DS and 3.5 yr old DD that I feel so different about and it's that feeling that eats me up.
My DS is not a badly behaved boy as boys go but he talks about superhero's and baddies ALL DAY and is wrapped up in his own world all about him.
Takes nothing and no body in and is very jealous of anyone else getting anything.
A day at the zoo yesterday for example. We arrive, he wants to 'go to the park'. Goes on and on and on. Then he just wants lunch. Goes on and on. Then he just wants to leave - argghhhh. 1.5 hr drive and £45 for that.
DD on the other hand is happy and engaging and excited about all the animals. She is happy with what ever is going on and just enjoys the day for what it is.
She cuddles us constantly, smiles most of the day and skips around talking to pretend fairies and singing to herself.
I turn to look at DS and he is pretending to karate chop a crow or his face is tripping him as we are not doing everything he wants.
To make my horrible negative thoughts worse, I had a 'surprise' baby 9 months ago - another boy! Although I have had no problems bonding with DS2 and I enjoy him, I feel very down about having another boy.
After reading your post im wondering - do we just have favorites and as a mum, find that in itself hard to deal with? Maybe all mums feel the same to some degree but don't admit it or even dwell on it as we are.
I do agree that spending time with your DD1 would help, not just her but you too.
I've been thinking I need to do the same as I remember having some lovely times with just DS1 and they can be so much calmer and nicer without their younger siblings around.