Re: conflicted about my attachment to my baby
I don't think this is known. But while some of the most fragile enzymes and immunologic components may hypothetically degrade under storage conditions, the basic make-up of the milk (fat, protein, carbohydrates, minerals) should remain the same.
Does anyone have any information on how much nutrients deplete from breastmilk once you express it and chill / freeze?
Congratulations, you're a great mom! Seriously, there is nothing wrong with your instincts, and no reason to feel bad or guilty about wanting to be with your baby and to feed your baby yourself.
In general I feel very possessive over leaving my baby with anyone.
That being said, if you want a date night you should be able to find a way to have one. Aim for small steps. For example, you and your DH and your baby and your MIL could all go out together. You nurse the baby and then hand her over to MIL who puts her in a stroller or sling and strolls around with her while you and your DH get lunch, with MIL standing ready to bring the baby in to nurse if she gets hungry. If that works out, maybe you'll want to do something slightly more adventurous.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"