OP, honestly you sound like you have some issues of your own.

Get yourself some therapy.

And no, do not contact the new GF. Your anger is not her problem. You may suspect they were involved even before the separation but a) you do not know that, b) as you said, you do not know what he told her, he may have said you had an open marriage or he was already separated or whatever, and c) so what? Calling her is stalkerish and weird. If you do call her and she has any sense, she will not have any interest in talking to you and you will just make yourself seem crazy and pathetic.

Get some therapy and move on.

Listen to your sister.

If you have shared credit you can probably figure it out from statements.

Confronting her won't get you the result you want, and might make things worse. File for divorce based on adultery, take half of everything he's got, and live your best life. When she realizes he doesn't have the big bucks she thought he had, she'll move on and you can give him the cold shoulder

He left 8 months ago and isn’t coming back. Your marriage is over.

Do not call his GF.

Yes, you have to ask yourself, what would you accomplish by talking to his new GF? Are you just trying to punish him? (Which may be fair, or not, but either way, what does it accomplish?( Are you trying to punish her? (Which may be unfair, and even if not, how does it make your life better to yell at someone else?). Are you trying to torture yourself? If she says, "yes, we've been having wild sex for years," will you be happy?

As for telling your children, DON'T. Even if they are adults, do not make them choose sides. It is fine to say "I am feeling hurt and betrayed by your dad" but leave it at that!

And honestly, People do change, and marriage is not a life sentence. I know it is painful for you to have such a long marriage end, but... these things happen. You probably bear some responsibility too for your marriage not being very happy.

But regardless, have some self-respect. Your DH does not want to be married to you anymore! Okay. His loss.

As your sister says, move on with grace and dignity. If he's a bastard, your kids and his new GF will eventually figure it out for themselves.

OP I know you are very angry. Talking to the GF won’t bring him back and it’s not productive at all.

Your children are bright enough to figure out what’s going on.

Take the high road don’t lower yourself your children will thank you years from now.

Today if you do anything make copies of all your financial documents everything from passwords to statements . Protect yourself first and foremost get a dam good lawyer before he does.

Why do you want to stay with this person?

File for divorce and move on.

If you do all that you’ll play right into your ex’s hands and allow him to paint you as the crazy one, and justifying his behavior. It is normal to feel rage, but find a better outlet for it. Your kids are old enough to make up their kids about his behavior.

Don’t lower your dignity by trying to talk to the girlfriend. What will you do with anything she tells you? Try to move on gracefully and make sure you get a fair settlement. Get into therapy to process your feelings.

Realistically, would therapy have even helped? Not much you can do about mismatched libido.

A friend of mine is going through the other side of this. His wife of 15 years informed him that she’s asexual and will never have sex again, after over a decade of almost zero sex. He’s been faithful up until now, but now he’s looking to either open the marriage or leave, especially since they’ve done years of therapy and this is what it’s lead to.

I'm sure your adult children can figure this out.

You don't have to lie to them. But you don't need to go on the warpath either.

Your sister is right.

Married for 28 years, two grown kids, and eight months ago DH dropped a bombshell on me, said he was unhappy and wanted to separate. I knew there were some issues, like mismatched libido (his high, mine low) and maybe we argue a lot about stupid stuff. But I thought we were committed. Anyway he moved out saying it was a "trial" separation but he wanted to be able to see other people and I should too, not that I wanted to. So then like a month ago he says he is not moving back, he wants to stay separated, and also he is dating someone seriously.

I know who she is an I am 99.9% sure he was cheating on me with her even before he moved out. I asked him when this started and he refused to answer, saying we are now separated and he's not going to let me give him the third degree. I am so angry I want to tear his limbs off. If he had any respect for me at all he would come clean about what he has been doing. He refused to do therapy with me or anything and I think it is because he knew all along he wanted out and has been sleeping around. I am still committed to the marriage but he is a lying s*ht.

I want to call his new GF and demand to know when this started. And I wonder if his GF even knows he was lying to me. Or if he has been lying to her, telling her our marriage was over before we even separated. I don't know if she helped him cheat on prupose or he lied to her too. And I think I should tell our kids exactly what their dad has been doing.

I talked about all this with my sister though and she is saying no, do not contact the new girlfriend, do not talk to the kids, accept that this is over and move on with grace and dignity.

Part of me wonders if she is right? But another part of me wants to burn his whole f*ing life down. He does not deserve to have his children's respect.