Alcoholism does not cause infidelity any more than it causes profanity.
Affairs are not just about sex they are about doing something "forbidden". It's an adrenal rush and role playing plus sex..
All cheaters are liars and he must have been pretty good at it.
It would seem He had planned this or there would not be a "fake FB account".
He is lying to you and to her (She does not know his last name). You may have all his passwords now, (and I say may) how do you know he will not just create a new account? Were there profiles on dating sites?...but even if not..This one has been going on for 6 months..that's no ONS.
He is only sorry he got caught-you in essence "forced" him to write that letter, he did not break up with this woman on his own. He wrote the letter to passify you and seemed not to care about this woman's feelings either. This woman who wanted more time with him.. he certainly was able to discard very quickly... (after 6 months, would she not ALSO not be devastated by him if she truly believed he was courting her??) Will she not want a confrontation of her own when she gets back in the country?
Did the chats indicate a serious flavor to the affair? Does he work? Do they have contact there or through computers at work?
I know this hurts, but you really need to take a serious look back in the memory bank and see if you honestly believe this was the 1st time this has happened. And it would take a lot more than His promise to convince me it would not happen again. Afterall there seem to be lots of secrets in this relationship.
It also would not hurt to be tested for STD's. Talk to your Dr. Some have a long dormant periods and may give a false positive. With the serious nature of HIV and other diseases out there today-infidelity risks his/your life.
What about Trust? How important is it? Use your brain and ask yourself the hard questions and answer them honestly to yourself. What I think or anyone else does not really matter. What can you live with or what are you willing to settle for..Can you not let resentments eat your lunch? If you were not particularly keen on sex with him BF, how will that be now? This will take time to process..In the meantime protect yourself financially. I fear there is a long emotional road ahead for you. Take the time to sort this out, you will need time to heal regardless of what path you take.