It's ok to sometimes wallow in 'This isn't the life I ordered!' Adoption tends not to be the life anyone ordered, or was the first choice, for any involved, and the taint of that sadness runs through every family touched by it.
I don't think it's proof that 'blood will out' or 'nature over nurture' or any other trite phrase people use. But sitting and saying 'This isn't what I imagined when I thought I'd get married, and have kids, and live happily ever after!' is necessary sometimes, and it's better to feel those emotions, and see them, and deal with them.
The truth is, no one gets the life they wanted, or very few people. You're hitting a really hard bit of the life you've been given, and that is painful and difficult, and especially at this time of year, it jars against what it looks like everyone else has.
People who have everything smooth are generally dreadfully dull. I bet you're really interesting! If a bit tired and sad just now. There's nothing wrong with a chunk of feeling sorry for yourself, from time to time.
I hope things feel lighter soon.