Hello again!
First of all, I'd consider telling him right now over skype. I'm not sure though, I hope others will chime in. On one hand, I can understand why something like this is better said in person, however, on the other hand I feel like I would rather know 5 days earlier and told over skype/phone than told 5 days later in person. And in some ways, I guess I imagine it would have been easier to control myself or feel safe if I got told via skype, because I wouldn't be able to act on my urge to hug her etc., I would be able to just click skype away if I didn't want to talk to her at that moment anymore, I wouldn't have to wait until she/I drove home to get to a safe place etc.
Furthermore, if you do tell him in person, try to do it somewhere where neither of you will have to drive afterwards (you mentioned telling him in a car) – this can be very stressful, you both will probably be crying and shakend and shocked, and in no condition to drive. Rather meet up with him close to his home/some safe place for him, go for a walk and tell him. After you're done, have the option of taking some sort of public transport or money for a taxi to get home.
As to what to tell him (maybe also write them down, so if things get too rough, you can read it or give it to him to read) – some things I wish I had heard (and she'd mean) were:
1. I'm so incredibly sorry, but I cheated on you on Monday or whatever day it was.
2. I know it's hard to believe, but I'm truly sorry and really love you and want to spend my life with you
3. It wasn't your fault at all, it was just my stupid, selfish and horrible decision which I'll never do again – I feel horrible for doing this to you... I have never been so ashamed or hated myself so much as know (but don't try to overdo it or manipulate him)
4. I'll work really hard and I hope one day you will be happy in our relationship again and perhaps even trust me at least a little bit
5. I'll be heartbroken , but I will understand if you break up with me now, don't want to see me now, don't want to make any choice, but I will continue to apologize, work on therapy and try to somehow make it up to you, and I'll wait for you and I hope someday you'll change your mind and give me another chance to prove my love through actions, not only words. I understand that you will maybe change your mind many times in the future,...., but I will understand and support you through all this, as much as you will let me... So go ahead, scream, call me names, I understand... (btw, don't accept any physical abuse or threats, don't accept name-calling for too long, I wouldn't accept a demand of a threesome or something sexual that you refused to do before – basically, tell yourself that you have dealbreakers too).
6. I'll be heartbroken, but I'll understand if you'll leave me to be with others... I hope someday you will give me another chance (btw, that doesn't mean you'll wait forever – again, dealbreakers, you shouldn't remain celibate for the rest of your life waiting for him)
7. I'll never drink alcohol again (I'd strongly suggest going so far – and I think he'll be triggered/remind everytime he sees you with an alcoholic beverage)
8. I've already arranged for me to start individual counselling
9. I hope someday you would be willing to go to couples' counselling with me
10. I'll get tested for stds (btw, you do know that you'll have to wait for around 6 weeks to get tested for HIV – I had to get tested, and they told me at least 6 weeks from the last sexual encounter)
11. I will give you passwords to all my devices, and we will install a keylogger so you will be able to see everything that I do
12. I will never talk to him ever again
13. I've written a detailed timeline if you will ever want to read it – it's all I remember from that night.
14. I will do a polygraph on my expenses so you will know I am, at least now, telling the truth and answering your questions. (yes, they're expensive, but it will be worth it for a chance to save your relationship)
15. We/I will tell anyone you want me/us/you to tell, or no-one, if you don't want to
16. my "friend" ____ was with me and she didn't try to stop me – I will not spend time with her again
17. Please tell me now and in the future what you thing might help you or what you want me to do
Ok, that's it for now, I might remember more later. I hope they are good advice and that I won't do more harm than good with my advice.
Oh, like others have said, don't minimize – don't say it was "just one time", don't say "but I was drunk", "but i didn't think anything would happen" – don't try to convince him that he should feel any less horrible than he will feel, by pointing out what you perceive or what are mitigating factors. Maybe, maybe, the time for that will be a long time in the future
One more advice – try to visualize what kind of questions and comments he will have and try to figure out what you will try to say or react. Maybe write them here and offer your idea of what you'll say. For example if he shouts in anger and hurt "How can I ever trust you again?! Why should I stay?!", "what we had wasn't/isn't special anymore etc.!", "Why did you do it?!" or sth like this. But don't be scared into not admitting things – DO NOT LIE OR OMMIT ANYTHING.
I truly truly admire you! You have more courage, integrity and honesty than my ex (and I still love my ex, so I mean that), and I really wish that she had been prepared to act in the way you are, and that she had been as remourseful as you seem to be. And that she were at least drunk, not completely sober. So even though I said don't minimize (and still say), to me, it would have been a slight consolation had she been drunk. Not a big one, but a small one nevertheless. So while I can't say what his reaction will be (and bear in mind that the first reaction can be extremely bad, but he might eventually, especially if you continue to fight for him, give it another try. Or vice versa... Just do the best you can from hereonin.), I will respect you a lot if you will tell him and be willing to fight hard for him. If not for him to give you another chance, fight for a chance to help him heal as much as you can...
Ok, this is starting to get hard to even think/write/remember of, so I'll stop for now.
As always, best wishes to both of you