I think there are several factors at play here:
When you fell pregnant the first time and it was discussed I am under the assumption that you were happy with the pregnancy? It was him that brought up all the negatives as in his threat of redundancy, finances and short time married, which in fairness are things to consider. Did you agree with him about these issues and feel like abortion was the best and most sensible option given the circumstances, or did he talk and talk and talk at you until he talked you round to agreeing with him? If it was the latter I can quite understand why you felt so angry and resentful of him. It is often the case that these things we consider big things before the abortion suddenly seem small and that they could have been worked out afterwards especially in the midst of grief.
Neither of you could predict how badly you would suffer with your grief and emotions following the termination. You of course knew and understood it would be upsetting, but there was no way to know the depths of it until you were experiencing it. However, I will say that if you felt pressured or coerced into a termination in any way than your reaction is quiet understandable.
Was he supportive of you during your grief? Did he hold you together and when you were falling apart? Did he take any accountability for his part in this? Or did he create distance and retreat into himself/work? Did he get frustrated with your upset, or not understand why you were so grief stricken? Did you feel supported or alone?
It strikes me that he may have agreed to falling pregnant again and makes me wonder if he felt like he now owed you a baby, or that giving you a baby was the only way to help you heal. Or if agreeing to this baby was in part due to his feeling guilty about the affair? It makes me consider if he really wanted this, or if it was his guilt, a distraction or a sticking plaster in the hope of healing wounds.
In your shoes OP I would need to think about all the above, I would want to understand if the affair was truly a symptom of struggles and a coping mechanism for him or if this is just what he is capable of and could have happened anyway regardless. Another thing to consider is the fact that he was capable of having an affair whilst you were going through possibly the most traumatic and difficult time. I am sympathetic that things will have been difficult for him too, and that seeing the fallout of the termination and the effect it had on you will have also been tough for him, but the answer to all of that was never to have an affair.
At the end of the day he was still unfaithful, he still betrayed you and he still lied to you….the reasons behind why don’t do much to change those facts. It also sounds like it was much more than just sex if this woman was angry enough to blow the lid on things to you and share pictures and messages. Indeed it sounds like she has been very hurt by him also and went scorched earth! What did he tell her, what did she think he felt for her if she is this angry and hurt about things? Because the way you described finding out from her, sounds very much like she was in complete destruction mode, rather than telling you because you simply deserved to know.
Food for thought OP, but I also wanted to say I am very sorry that you are going through this 💐