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You are not the first to ever seek advice in here (I don't know how helpful help from people in here has been to previous people) and when I read threads like this I can't shake that feeling that there is always an important piece of information, a key piece of information, that could help better understand the situation that is being omitted.

All I can say is, get professional help! And as quickly as possible at that.

Depression's a bitch, and it's hard to leave that behind without any kind of help. Worst case scenario you'll need medicine to get better, but start talking to a professional.

That's the best advice you'll get from anyone around here. Take it.

Depression is not a joke. It is something that effects a lot more people than we think. It is taboo to talk about and seek help but one must do so. It seems like you had your early version of mid life crisis. Usually it is said to occur much later in life but since you met your life partner very early and had a long term relationship. You stumbled upon it in your mid 20's. The good thing is you are acknowledging the problem. Now you can start seeking help from professionals and people you have in life that care about you. Hard to say if things will go back to how they were before but they can only get better if you think you've hit rock bottom. 

Cheers. 

Are you feeling depressed because of something that happened or did it just come out of nowhere?

I don't think I've ever experienced this much regret before so my advice is probably next to worthless but I have a few questions ...

How much confidence do you have for your self and for your beliefs ?

What is your current lifestyle and where do you think your headed in the future ?

What made you get into depression in the first place ?

I think you might be suffering from existential crisis ...

How are things with the other woman?

Also, why do you think you're unhappy? What is it you actually want in your life at this point?

Just because you were happily married before doesn't mean that there's not something else you're looking for now, even if you're not sure what that is at present time (or maybe you are but just don't want to admit it?).

You're still young so there's plenty of time to shape your life how you'd like.

Maybe you felt like you were not doing anything worthwile? Like you were wasting your life away, and was in need of new experiences? That might have triggered your depression.

List out your problems, your status at work and find every options you can take to further pursuit your interest in life.

Did you ever do what you want or did you follow what other told you to do, school job marry what not.

Have you ever pursed your dream, what you fantasied about what all other said impossible ?

you sound like someone who never got to live risk and do what they wanted to.