First, I'm glad you found your way to this site and I hope you stick with it, as difficult as it might be at times. People here, especially in this forum, do not mince words and tell it like it is. But it's done in order to help, not to hurt. Too many WS's and AP's don't want to hear about or deal with the hard stuff, but it's so necessary. You should also definitely check out the healing library, there are many articles of interest there that may be of great help to you.
Now, that being said, I do have some rather tough questions for you. First, WHY did you continue to email the AP after the "courtesy" email where you just wanted to make sure she was okay after the accident? Or why did you even email her in the first place? Why were you so interested in her personal life, especially the state of her marriage? However "horrible" it was or is, if that was even the case, actually, it was NOT, in ANY way, your concern or business and you should not have concerned yourself with it at all, period. Your concern should have been your wife and her feelings and needs. Were you trying to gain attention for yourself by playing the KISA (Knight in Shining Armor)?
Second, why did you even begin texting with her? What possible employment reason would there have been for that? And why did you allow it to "quickly become sexual in nature" as you've stated? You should, first, have never allowed it and, second, stopped everything right then and there. You write as if you had no control over any of your actions in this regard, as if things just happened on their own. But you had choices at every step along the way, you need to take responsibility for those choices that led up to your infidelity.
Third, when called upon to explain the high cell phone charges for texting, why did you lie? Why did you look your wife straight in the eye and lie about the nature of your relationship with the OW? It's certainly no wonder she doesn't trust you at this point. Can you understand where she's coming from? You need to examine why you're able to lie so easily and why you seem to automatically resort to lies in the first place. I was like that once, also, for me it was major FOO (family of origin) issues. Once I got to the root of those and worked through them, I was able to become a different person, which included no more constant lying.
Fourth, why did you never try to work through your previous instance of infidelity? Just because time passes, doesn't mean the issue goes away and isn't affecting your wife. And why have you never sought therapy for the issues you admit you have (fear of aging, anger and esteem issues, etc.)? It is really critical that those are all dealt with or things will not change and you will end up doing this again.
You say that your and your wife are both going through hell right now. Correction. Your WIFE is the one going through hell, thanks to your actions. It is up to the WS to do everything they can to help the BS, including being totally truthful and honest in telling the whole truth about the affair and answering any and all questions fully and honestly. YOU have caused this situation, so you must be patient with the BS and give her what she needs, even if that means not being around her for the time being.