I have to lol at most of this. I got to like the bottom of the 2nd page then realised there were 2 more pages til I could comment.
I'm gonna refrain from actually giving any advice. Why?
Cos OP, you're shooting down everyone who's giving you advice. Then you're going on and on about stuff. It doesn't matter if your ex was flipping mother Teresa, she left. That's it. Get the hell over it. She's probably been cheating on you with the guy as well, risking passing skanky stds back and forth(because yous have only been relying on the pill as contraception, right? ha-how retarded), and hey did you ever find out what was making her have mood swings? it was probably HER PERSONALITY.
Wait, I could say something a bit contructive. I hope you take things slower and more carefully next time. Not just for the woman in question, not even for the kids you may or may not end up having(if you have any more). But for yourself. Taking a step back right now, it seems you jumped right from one woman to the other, with no gap in between to sort yourself out a bit, and even tho there's no indication of time, you might have rushed into being in a relationship with the recent ex. I know that sounds a bit iffy, but I can't tell if yous went on like casual dates before deciding to be intimate. I'm sorry if I'm wrong, and that yous did take it slow, I just hope it's useful for the next time.
Rather than thinking of it being *oh Karma cos you done it and now it's being done to you* or something similiar, perhaps you should try to find something to learn from it all.