If you had been married for 25 years, you must be in your forties or fifties. I was married to F-Nuttz (you can guess for what the "F" stands )for 35 years, and "together" for a few more than that. So you are now crazily bobbing along in the same boat I was not too many years ago.
A few thoughts first.
He lied to me from the beginning of all of this about him. I lived with fear that that child and the mother would one day ruin my marriage and my family and that is what has happened.
My first thought: neither she nor the child ruined the marriage, although on the surface, it was a catalyst. He lied from the beginning. Many of these liars were liars to begin with (your Nuttz); othersl, once they start to lie, even if they lie badly, find it easier and easier (the F-Nuttz). Had you known he was a liar, would you have married him? Most likely you would have, as we caregivers believe we can change others, given love and time. We are forgiving as well.
The issue was, and is, his lying. More on that later.
The other point has to do with fear...more on that later also.
You fear being replaced.
You will not be. Your daughters know full well who loves them, and has from the beginning. Too bad our adult children behave like they are still, well, children. OW has nothing attached from a lifetime to the girls, and they might see her as "fun." That is the generation, I think, rather selfish. I wanna have FUN, and don't tell me about your problems. My adult son I love very much, and he loves me, but he doesn't want to hear about of it, never did.
The cheaters and liars are irresponsible; they have convinced themselves whatever they do is okay. The F-Nuttz called his adultery "Stepping Out." That will be the name of my book.
Now then. I know the pain you are in, we all know the pain. Lean on us here. Too, if you have not talked to your doctor about the lack of sleep, the depression, don't be afraid. Even I, afraid of all medication, took xanax at night and took anti-depressives. Too bad I didn't know them about Cymbalta! I am not ashamed to say that I need medicine to help with chronic depression, mood disorder, whatever one wants to call it. In fact, I am proud to say I can now ask for and accept assistance. Let go the control obsession.
I am going to post an essay by SerJR; I do this periodically. Read it, copy and print it. I still read it.
Time. I heard the same thing from our fellow SIers. But truth it is. I visuallized the forest, and peeks of sunlight coming through, tromping on. Feel free to PM anytime.
You will be okay.