Okay I'm going to start by saying I've been pretty much at every point in this situation on both sides, short of leaving my DW (she did leave me at one point though), so I would like to think that I talk from some experience.
If there is an OW (and at the beginning I suspect there wasn't, but that may have changed) then it's not the problem, however your DH may see it as the solution, or part of a solution. So if you're going to fix this (and I do hope/believe it can be fixed) then there is no point on focusing on an OW. You need to speak to your DH in a frank and open manner, and despite what you might want to say, keep confrontation to a minimum (more on that later).
Stress and depression of one sort or another are likely to play a big part in all of this, including but not exclusively:
Lack of self worth
Feeling like a passenger in life
Carrying the financial burden of the family
Feeling trapped in job to carry financial burden
Loss of the sense of self
Inability to communicate with DW or anyone else
Fear of ridicule for mentioning any of the above
Sense of getting old
Feelings of inadequacy in work, and life acheivement
Loss of connection with family
Guilt caused by all of the above
Now those are all quite general negative feelings and don't need any specific situation for them to arise. There can also be more specific elements in your life to add to those things like financial or workplace stress. Unfortunately depression is self fulfilling and becomes a vicious circle, and sometimes people just get sick of feeling down all the time, and perhaps they can no longer "do this".
If someone is living with all that on their mind, an easy way to fix it is to walk away from their situation, and all the negative things that go with it. Unfortunately your DH may have associated you with some of the negative aspects of his life, but that doesn't mean it's personal, but you're the other adult in the house, and the natural focal point for some of that negativity over a period of time, especially if small issues get turned into major issues in the mind.
So someone could be forgiven for doing the following:
Move out of house that's associated with sadness and pain
Return to childhood area, because life felt better back there
Get a new house/flat and fill it with only positive things
Get a new job
Hang out with old friends, because life seemed better when they were in your life
And then once you've done all that, there is something missing, a SO, and at that point it's very tempting and easy to fill that void with something new, but vaguely known. So an old flame, or someone you know a little, resulting in a new exciting relationship in which you have all the thrills of falling in love to do over again. All of a sudden it appears that someone has managed to turn their life around and found happiness.
Seems like a good move on the surface doesn't it? You get to sort your life out and start again, makes perfect sense. Except of course that along the way there is a massive fallout with friends, family, children and of course the estranged SO. And it's probably likely that this situation will enter a new cycle and your DH will end up doing something similar again (the grass is always greener etc...)
For my sins I realised what what was happening in my relationship, managed to swerve getting involved with someone else (as an escape I suppose) paid for therapy to try and make sense of what was happening. There is always an element of depression in the shadows and I have to work at staying positive, but by doing so I minimise the negative aspects that may otherwise try and escape from. But there are still bad times, however I never blame my DW.
So what you essentially need to do is work out if you can be happy together, figure out what was/is making your DH feel like he is and try and work those issues out. If you can do that then you might have a shot, but it's also very possible it's gone too far and it can't be salvaged. But if there is an OW involved I can assure you that's the result of what's been going on and not the cause.
Depression in it's varying forms has common themes, like paranoia, lack of clarity and irregular reasoning. So it's not impossible to turn someone your love into your enemy, even if it's only in your mind.
The absolute worse thing you can do is confirm all of the negative thoughts and ideas your DH might have about you. So as hard as it might be you will need to allow your DH to talk if he is able, and reassure him that you can listen, not judge him and slowly you might get to the bottom of what's going on. It could be that things that barely register on your radar have become major issues for your DH. The first step is to change those things, and then when you are both in a better place mentally/spiritually you should be able to deal with them with the benefit of clarity. I really don't know how to suggest you approach sorting it out initially, and making the first move is likely to be the hardest. I would say that you should act fast, because if there isn't an OW right now, there might be one very soon and that might seal the deal not in your favour.
My DW was certain we were finished, telling me bluntly to to the point of being hurtful. But she had a change of heart when she supported me through an unrelated trauma that occurred out of the blue. But I also agreed to give up everything except us, and also to get some professional help.
Doing a runner on your family is not the only way to change your life, as can be seen from some of the other posts in this thread, so you do have a chance, but things will need to change in your world in order for both your lives to converge again. For both of us being able to do what we want in our hearts, not feeling trapped in a situation (not a relationship specifically), and generally trying to stay happy have been the key things to staying together.
I'm not here to blame it on you, because I was in your position and I know exactly how you feel. That feeling of sickness that means you can't eat, to punishing yourself by not eating, the anger, the shame, and generally wanting to close your eyes click your heels and open them again back in the world you thought you lived in. But at the same time I've been the provider and just another guy having his "man crisis", tempted by a new life and escaping all the things that I felt were dragging me down. So I can appreciate what your DH might be feeling and why he has taken the choices he has. It's a shitty situation and you're holding onto the shitty end of it, I realise that a few posts on a forum aren't going to fix or shine the light that leads to a happy ever after, but I felt my experience might be able to help someone.
Good luck to you and your family.
It's not the done thing on here and you're not really laughing are you?
If that's what u all think then I need to find out ASAP.. It will help me move on do u think??? I do think we had a few unhappy days picking at each other etc... But It just became a routine and how we were.. I never thought he would just walk out.. We have been through so much together and had so much planned.. I suppose its all in black and white really that there must be someone waiting in the background for it all to start... I just can't think when he would have met her.... He wasn't on Facebook.. The laptop hasn't come out for months.. I had his phone bill last week and nothing unusual.. I just don't get it