Hi Steph,
Well, being pregnant can definitely cause those emotions, or even just make the emotions that are already there a whole lot stronger.
I know that it's hard to get over an ex... after all, a little bit of your heart is his!
I think that it is probably partly what you said (that there was stability before and your life is uncertain now etc), and partly just because we all tend to think about what could have happened or what we 'lost' or what we could have done differently.
Are you with the father of your baby? If you are, maybe just trying to remember why you fell in love with him could help.
What helps me to get over different guys is to, when a thought about them pops into my head, I 'throw out' that thought and think about something else. (Sometimes you'll have to do it 60 times in a minute.)

But maybe that would help.
I hope that your emotions settle out soon! It's no fun to live with super strong emotions!