From what you say in your OP, your boy has been badly bullied at school, which is why he can't go.
I'm so sorry you are going through this nightmare. There are many people on here who have been through the same - do look out some of the threads on 'relationships'. imo it helps to know you are not alone. Or, it helps a bit, but not much in the initial stages when the pain is absolutely immense.
I really don't know what happens to these people who abandon home and marriage for another relationship, kicking into touch those left behind. It's as if they're possessed: someone on another thread a long time ago said the husbands who had done this had 'gone over to the dark side' and I can't help thinking that's an apt description. It's as if they have a total personality transplant overnight...
You say it is not good for the kids to be with a drug addict - is the OM a druggie? Or do you mean your wife? If she is taking drugs, that could account for the dramatic change in character; though, sadly, the majority of people who do this aren't on drugs but are high on another type of drug...
The pain must be immense, as she was your whole life. It does look like you made her your reason for living and did everything for her. This is not good for her (and, of course, you) and perhaps later on in your recovery you may be able to address possible codependency.
I do hope support is in place to help you (and of course your kids) through this awful time. At the moment your lives have been hit by an explosion and it will take a while to see where all the parts land. Try to be patient.
It is not right for her to be coming and going when she pleases. YOu really must put a stop to that. Does she have a key? I would also be very wary of her taking one/some of the kids - as someone said upthread, it is very hard to get them back if she does this.
However, she has abandoned them and it may not be as hard to get them back as it is for some, but nonetheless you don't want the kids going through any unnecessary added trauma and upheaval. You can get a free first half hour with a solicitor to set out your case and see if you are eligible for legal aid: it's amazing what they can cover in half an hour - you really must get this ball rolling. It is very disruptive for the kids to see her every day. Apart from anything, the upheaval for you is not good for them. YOu need to get steady and try to find your footing after this awful blow.
Keep going. YOu can do this. The prozac takes a while so hold on. Please make sure you get out in the fresh air and eat healthily (if you can't face much then try to cover the main food groups eg protein [nuts?], fruit/veg, calcium [cheese, milk], carbs [brown bread etc]) Try not to eat crap because it makes you feel much worse. Make sure you get enough sleep. Exercise is an amazing anti-depressant so try to get out for even a walk because it does wonders. You have to attack this from all sides and practical steps to protect you physically are far more effective than you may realise - you need to be strong, not just for you but for your kids.
You sound amazing, dad.