Tapper you say you didn't want to give birth to two teenagers, it may not be what you wanted but due to circumstances and choices made by you and your dh it is what you've got.
If we can't change circumstances to suit us we have to either change to suit the circumstances we find ourselves in, or, walk away and find a more palatable set of circumstances.
To blame and resent others for the situation you are in is totally unfair on them and to an extent on you, it is corrosive and it won't be the circumstances that have a detrimental effect long term but your reaction to those circumstances.
The analogy to people staying at Christmas is too simplistic, people at Christmas are invited and there is a start and end date to the visit. Your step children are not potential guests to be invited when it suits, they are part of your dh and due to circumstances beyond their control and through no fault on their part they needed a home with their father and step mother. Once they are in that home and part of that home and family there is no definitive end date, it is their home and their sanctuary from the wider world, or it should be.
There are innumerable people who post on Mumsnet, and in the wider world who could regale you with tales of their lives and the unwanted situations and events that they have had thrust upon them through no fault of their own. Life doesn't often turn out how you want, more often than not things happen that you didn't want, accept that, stop railing at the perceived injustice of the universe to you and adapt to the situation you are in for the benefit of everyone, including you, or walk away. Life is hard and unfair all any of us can do is live it to the best of our ability and make it as happy for us and those around us as is humanely possible.
Only you can change the way you feel or react to the situation you are in.