Winnie, I'm so sorry you are going through this.

May I ask, for context, are you separated or divorced and are you living in separate dwellings or in the same house with your ?ex and young children?

Ok thanks Winnie.  I think a little bit of deconstruction is needed here.  This is not a good story and it doesn't have a happy ending unless you get out in front.

We have been separated for almost 5 years.. I finally got the courage to leave eight years after finding all of the classic evidence, gay porn, Craigs list hookups, etc..
He is gay but still in the closet.
In those eight years the topic almost NEVER got brought up.  I suffered in silence, because he said if anyone ever knew he would kill himself.  To this day, he still does not admit it.  The closest he has come is to say "he doesn't want to be gay".
He is gay and still in the closet and you have chosen to hop in there with him, because he emotionally blackmailed you with threats of suicide. So he is abusive as well as a lying cheater.
   During the last five years, I have kept the family intact.  We still behave like a family we just lead separate every day lives.
That is not behaving like a family.  That is shielding your gh from consequences.
 My children know nothing of his sexual preference.
Again, you have stayed in the closet and shielded your gh from consequences.  Children - especially adult children - deserve to know the truth, no matter how challenging it might be.  Otherwise how can they make real, adult, fully informed choices about how they choose to relate to their parents? 
 But now..... he has started dating a woman!   He is introducing her to our children and acting like this is going to be a committed relationship.... How can he do this???
He can do this  because he is still a hugely entitled, abusive, lying cheater and has not experienced any serious consequences from his actions from you yet.  He even got you to move out of the house so he didn't even have to do that!  
  I feel so angry and jealous and don't want her "filling my place" in the family.... all the while, me keeping his sick secret....   I need advice 
And so you should feel angry and jealous.  But ... he has already put her in your place, introducing her to your children as though its a done deal.  He is acting like he is divorced and free, and also as though he is straight.  So what are you going to do about that?  Suggestions:
1.  Divorce - get a kick-ass lawyer and get out in front before he hides all the money and your financial future is destroyed and you lose everything.
2.  Tell your children the whole truth - they are old enough to know.  
3.  Stop keeping his sick secret and Don't keep any more secrets 
4.  Find a good therapist for yourself 
We really don't see each other much, unless it is a holiday, vacation or some other special family event.
You have worked your tail off pretending "its ok, we just grew apart"  and making nice family celebrations that he turns up to.  He has had everything his own way for so long that he has forgotten he still has a wife. 
. I have just assumed he lives his gay private life and I don't have to deal with it.  The last five years of my life have been blissfully peaceful and neither one of us lost our family because I made a conscience choice to not abandon him.
He is now abandoning you.  He is in the process, quite openly, of replacing you.  Sadly there are no prizes for denial, even if it is peaceful and if you think it was for the children.   As I say, I recommend getting out in front with the divorce before he hides all the money.
 I actually do worry a lot about him and what a toll this takes on his mental health.
Umm yeah, the old fragile mental health trick.  His mental health is so bad that he has got himself a girlfriend who he is happily introducing to your children.  His memory may be faulty - he has forgotten he is still married - but his mental health sounds just dandy.  You are in danger of losing everything if he decides he wants to marry this person. 

I hope your finances are rock solid and all in your name because if they are shared, you are in for a shock I fear unless you lawyer up and get that divorce. 

And if he ever threatens to kill himself again - call an ambulance every time and let them deal with it.  If he means it they can help him and if he doesn't mean it he won't ever use that threat again.
.  I can only say this now because it has been 13 years since I found it.  In the beginning I was a puddle on the floor and completely consumed with disbelief and anxiety.. 
Winnie, I am sorry but I think your troubles are not over yet.  Please don't melt again into a puddle.  Please tell your children the truth and secure the best lawyer you can as soon as you can.  Get this jerk out of your life once and for all.   

I'm sorry if this sounds harsh and I am not being mean, I am being realistic.  Its a story I have heard too often.  Good luck.  

Cheers Elle. Hard truths sre ... hard.

Winnie, I hope you don't think I am blaming you in any way for making the decisions you made for your family. We discsrded spouses are under tremendous pressure to make no fuss and just shut up and get out of the way.

My 25 year marriage had been abusive for 15 years before my eyes were opened and  I found the courage to leave and divorce my gxh. I was sick of lies and facades so I decided to be truthful without rancour with my young adult kids.  It paid off in honest and loving adult relationships with them.  They were able to make up their own minds about their father armed as they were with truth.

I fully understand how hard it is to see these men for who they really are - selfish, cruel, massively entitled, lying cheaters who grind us down to dust with their emotional and psychological manipulation and cruel disregard for our feelings or our years of devotion.

I am saying you are not safe once he has replaced you with another woman.  So take steps to ensure your financial and emotional safety as he begins this new chapter of him discarding and erasing you.

They are ruthless. They live by lies.

Thank you all.. I can hear all of your pain...  I have so much pain over the last 13 yEARS.. it is unspeakable...the bottom line is... these men are not monsters... they are the father of our children.. our providers and our caregivers in time of need.. atleast mine was.. he was my rock and my savior in time of need... I have had my own demons, which have not been easy to overcome.. my husband never failed in trying to help me... maybe it was a coverup or a mask, or a narcissist characteristic  to hide his own secret...  whatever the case may be.. he was there for me in a time of need... these men are not monsters... they are trying to be happy just like the rest of us.. whatever secrets we may have...  I have suffered a great deal in a mis matched marriage... and he has suffered from the hands of nature.. I still love him...

You do not have to believe that your husband is a "monster" but I suggest that you accept the reality that you and he have been separated for years now and he is moving away from you, although not in the direction you envisioned. Introducing a person to our children means he considers it serious.

He may have been loving and supportive for you in the past but what are you getting now from prolonging this marriage? If you have not consulted an attorney to discuss your situation I strongly suggest that you do. You don't have to hate him but you do need to love yourself enough to protect your finances and future comfort.