I am going out of my head and think I am getting depression.
I think my wife is suffering from a mid life crisis and its wrecked my life.

She filed for divorce (married for 21 years)
She swears a lot and blames me for everything
She has taken a new lover aged 20 - she is 43
Is secretly seeing her sisters boyfriend without her knowing,
Wears totally revealing sexy clothes
Spends money like no tomorrow
Takes dirty pictures and videos of herself and sends them to men over the internet
Lets her sister rule the house and dish out dicipline to the kids, threatening to hit them
Contradicts herself all the time,
Says she doesn't love me and doesn't want to be with me

My wife was a totally loving and caring person and now its as though she has one totally crazy.

Help. What can I do

Has she seen a Physician? Will she see a Physician?

Has she seen a Physician? Will she see a Physician?

No and no. She would probably laugh at anyone who suggested that she has it. She would put it down to me, might as well I get the blame all the time.
She just fired the 'filed for divorce' at me on boxing day 2012. We never spoke of any problems or sat down and talked about anything.
Now we never speak to each other, never text or call. I tell her only on a Saturday and Sunday that I will be down to see the kids at 12. She keeps firing that she has the kids 24/7 at me despite the agreement we made and the solicitors that I seethe kids at weekends. She keeps saying I don't make an effort although I'm following the agreement to the letter.
I think I'm going to see a doctor because I think I need councilling to help me cope. I love her dearly and she knows that but chooses to ignore it.
I still can't get my head around it. How can she turn her back on a 21 year marrige and forget it ever existed and continue to have sex with boys of 19/20, the same age as one of our sons. We have 8 children together.

No way would she ever think of going to seek medical help

I would start protecting yourself and prepare yourself for the inevitable court battle that will be your divorce. It will hurt but doing this will help you in the long run. Make sure that this is only HER money that she's spending and not yours.

There could be a multitude of reasons as to why this is happening. It appears that only she will know how or why. So, until you talk to her or maybe her sister/mother, you will be in the dark too.

Making sure you are legally and financially protected is a VERY good thing right now.