A- bet you dollars to donuts he's been with this woman since before you separated. That she is the REASON for the separation in the first place. Statistically (I've been here a long time) the odds are about 100 to 1.
B - His comments earlier about not being happy for 3 years, and his bitter comments to you at the time (instead of ILYBINILWY) - sounds a lot like the typical comments of a guy who has been playing around the whole time. Kind of like the guy who started his affair before the wedding (!) or the other guy who was having multiple affairs throughout. I may be wrong, but you may want to do a little sleuthing to see if my instinct is correct. Do you know this OW? Is it possibly someone he has known for a long time? Could he have started with her that long ago? Did she ever come out of the bedroom? Does she even know you are pregnant?
This might be one of those rare instances where contacting the OW could give you valuable info. I say this because, your choices might be a little different if this is just a case of him starting to date after you were separated, and being a little torn, versus a case of he's been cheating on you since before you married him and you are just now finding out.
