HomeBirthMummy I've no idea whether you are still reading this thread, but this is my take on it.
At the moment, like Counting says, you are still reeling from one of the most awful weeks of your life. Sadly, from what you've said about losing your Mum and your unhappy step-mother experience, you've taken a lot of knocks in your relatively short life. It seems so unfair.
I hope you haven't given your H a decision yet, but I understand where you're at. If you had thrown in the towel last week, you would still have been hurting just as much, but with less information than you're going to glean over the coming weeks/months. I hope that you are saying to your H that you simply don't know if you can forgive and get past this. Your final decision should be in abeyance. You do not have to decide now - or anytime soon in fact - if you can re-commit to your marriage. He should respect that too.
What I hope is that you'll find out enough information with which to make an informed decision about something that is so important.
If you're going to try though, can I counsel you to insist on certain things that are going to help you in the coming months?
It is vitally important that your H tells the OW the truth - about you, his feelings for you and about your marriage. She should be left in no doubt at all why he is committing to you and your marriage. Given the way this ended between them, it is likely that she will delude herself for years to come that he would have left you, that he's only staying for the children/pressure from his large moral family - you name it, she will comfort herself with this. It will not help you come to terms with this if you feel that there is a woman out there who believes your marriage is a sham and your H is staying for the wrong reasons, so get that closure now and control the message.
Secondly, he should cease any friendships forthwith with anyone who was no friend to your marriage. This will probably mean changing jobs/role.
He is doing some of the right things - the bed, the STI test, the counselling (on his own, remember - this works best) the GP appointment, but he is also building a few smokescreens that in time you will see through, just as others here have done.
I suspect his chronology is flawed somewhat and it is more likely that he started to complain about you/push you away when he first had designs on the OW. I suspect he might have felt a bit down and overwhelmed by the responsibility that 2 small children bring, but I would doubt it was proper depression. I don't doubt that he was under stress and this explains the weight loss, but very often that stress is caused by leading a double life. Stress and depression are very different.
It doesn't surprise me that he is putting forward reasons for why he was vulnerable to an affair, but I think he is evading personal responsibility hugely. The unpalatable facts might be that he was immature and selfish, was feeling a bit bogged down and a developing attraction for someone else gave him a buzz. When she appeared keen too, he told himself "why not?". One of the most revealing questions you can ask him in the coming days will be "How did you give yourself permission to do this? What were your thought processes?"
He has behaved no better and no worse than OW - they have both behaved abominably and I don't think either of them have any illnesses or excuses for what they have done. Her H might also be demonising your H for all this too - and if she was using a "drunk" defence in the early days - he might still be locked in that denial. Hopefully he will be wise like you and see through it.
Your story just proves the wisdom of verifying what you were being told. The sad truth is that none of this would have come out - for either you or her H - if you hadn't both done your own digging and verifying.
You are still going to be hearing a fair few lies for some time to come - some will be lies so as not to hurt, many of them will be to save his own skin and some will be the lies he told to himself. Do what you did last week throughout - verify and insist on honesty when things don't add up.
I understand why seeing your H a broken man will have tugged at your heartstrings. You love him and your basic sense of humanity reacts instinctively when you see a loved one in pain. Please don't go into rescuer mode with him though. If you suppress your hurt, your rage and your sense of loss, you will regret it further down the line.
There's no way of telling from the facts on here whether this man is worthy of a second chance. There's a chance that if he enters freely into proper pyschotherapy with a very challenging counsellor, he can change. However, there are some nasty twists to your tale that are going to be particularly hard to overcome - the disparaging you to friends, the violation of your homes, the acceptance that he would leave you, the OW ending the relationship, the "flight" mechanism after disclosure, the ridiculous lies - possibly even now the smokescreens about depression. The length of your marriage worries me too - when a man is unfaithful this early on, it is not a good portent.
So, in the final analysis, wait and see, ask questions and verify, challenge denials and delusions and take your time. On the hugely positive side, you are young enough to start again if you decide you cannot get past this and forgive.