How thoughtful of your DH to tell you how unhappy you've been for years, as if you don't know your own mind, silly little woman. Kind of him to transfer his guilt into you.
How you handle this now will affect you and your DD for years to come. So write out a plan of the perfect divorce. What it looks like, and what the outcome is.
That will help focus your mind while your emotions are all over the place. Return to it every day and amend where necessary.
It may also help to write down how you hope to feel, and where you want to be emotionally, in one month, three months, six months etc. That will give you hope, that these stressful and encompassing emotions will pass.
Next, even though you love your, soon-to-be-ex, DH realise that soon, you will recognise what a blessing and opportunity this is.
You get to live your life how you wish to. The new G will spend her evenings, when your (ex) DH is working late, worrying about who he is with. Also, the gloss of new relationships fades fast, and they will both be left with the legacy of how their relationship started - as a secret, lack of honesty and respect (on his part).
The GF should see what's in front of her nose - when a man shows you who he is BELIVE IT.
Your DD needs you to show her how to handle emotional turmoil with dignity, fairness and healthy self respect. My mother didn't when she divorced my father and the repercussions are still being been felt 25+ years later. Cry and rage in private to your friends. I'm not suggesting you don't cry in front of DD, but save the raw emotions for a friend, family member, the dogs.
You have got this, you can do it. You are a woman, stand up and shine bright.