When he suggested that you get a new SIM, basically he was asking if you were up to something beyond innocent. When you got that SIM, you confirmed that you were prepared to do things with him (your XH) that your DH must never know of because it would hurt him. Now whilst you by the sound of it only meant conversation, he might well have understood something else. I would not be surprised at all if you found him at your door at some point.
You were wondering about why you felt as you do, why the emotional rollercoaster and all. Well here's a very speculative theory, but perhaps you can find something in it that rings true?
You said he treated you really badly when you were married and that when the relationship broke up, you were left with zero self-esteem. I'm wondering - why the low self-esteem? Probably because he manipulated you into thinking (or feeling, at least) that you were worth nothing. Maybe he was unfaithful, but instead of you thinking what a bastard he is, he manipulated you into thinking it was your fault - obviously you weren't good enough for him or he wouldn't have had the need to find other women. Or maybe he hit you and managed to make you think that it was your fault, as you provoked him. Or maybe he just spent all your money on his own interests and never spent time with you, and at the same time made you think you were at fault because you were too boring to capture his interest.
So then when your marriage ended, you spent years working back to some semblance of a normal life and self-esteem. But perhaps he hurt you so profoundly that you always kept a nagging doubt - perhaps it WAS my fault? Perhaps I should have done something differently? Perhaps if only...? So though you got on with your life and put that time behind you, you never became confident enough to realise - NO, HE was a bastard! Otherwise why would you have sought him out again?
So now you get in touch with him and he pours his feelings out to you. He admits his guilt, says he did wrong... he finally assuages that doubt. He tells you: No, it was not your fault, it was my fault, and I am so sorry. Obviously your emotional state becomes precarious! The relief... he who destroyed your self-esteem is now telling you it was not your fault. Isn't that the best reassurance ever?
BUT the problem with this is that you are again making your emotional well-being and self-esteem dependent on this man. This man who has proven not to be trustworthy - proven it to you, and to his wife. You are letting his flattery make you feel good... Why should it matter to you what this man thinks and does? He says he repents... didn't he say that, perhaps many times, while you were still together - just to go on and do whatever he did again a bit later? Why do you believe him now?
Maybe now, thirty years later, is the time to finally get over him. He hurt and denied you so much that your self-esteem never really recovered (as you wrote). Now you have it in your hands. You can deny him. YOU can control what happens from here.
Why not get your DH on board? If you put it like this: "I have been in touch with my XH by e-mail. I thought it was just friendly chatting, it was interesting to hear what he has got up to, did you know that he has been married for thirty years now and has two adult kids? But now it seems like there is something else happening. He has lied to his wife, he has asked me not to tell you that we have been in touch, I think he wants to meet, it's starting to creep me out! I don't know what he's getting at but it is a bit scary. So if he ever calls, can you please tell him I don't want to talk to him?"
I'm sure that once your XH realises that you are not the easy-to-be-manipulated young girl anymore, but instead a happy, self-assured mature woman with no interest whatsoever in having an affair with him, he'll lose interest in you pretty quickly.