What can I say to try to ease things a bit? You are quite bright enough to have thought of all this before. Perhaps being told by someone else may help.
Children reserve a special place in their hearts and trust for parents. With good parents that works out well and the love and trust is totally returned, and also used to guide the child into being an adult.
Sadly we do not always get the parents we deserve, I didn't for one, and you did not get the mother you deserve either. A major long term downer.
You got a mother who was not loving and even managed to hurt you when not even trying to. A selfish woman who now, despite having hooked up with another, wants more than her fair share of what she and your dad had together.
On top of that she is having a baby. OK, on the surface one might think that she was unhappy with you and was trying for better. This of course is rubbish.
One standard way some women have to 'cement' a relationship is to get pregnant and have a baby. I guess they think it is something that will bind the man to them. Frankly I feel sorry for this new life when it arrives, it will not be loved any more than you were and she will probably be just as incapable of mothering it properly as she was for you.
Sorry to be so blunt, however I'm trying to get across the idea it is no lack in you that makes her behave as she does.
Am I making sense?
I'm sure you dad finds great comfort in your presence, even if you two might find it hard to get a decent place to live if things go badly.
Thinking of this all the time is not that good for you, which is why I asked if you have 'mental retreat'. An imaginary place where you can step out of the worries into a different world for a moment. Amazing how much perspective that can give.
Maybe for a few minutes imagine you were at Haight & Ashbury in the late 50's or 60's.
You might see Janice in her Mercedes Benz:)