Hello NoCapes. Sorry to come to the thread so late, I've been reading it when I can today, but not had a chance to post until now.
Your dilemma really resonated with me, because when I was 26, I was in a very similar position to you. But I didn't have the courage to leave, and I let the whole sorry saga go on, until I was 37. My ExH sounds just like your "D"P. Finally, at 37 I just snapped and made my plans to leave. By then I had 2 kids by him, but I just couldn't stand it any more. He had tried to seduce all of my friends, my sister, my Mum (yes, my fucking Mum) and also shagged by Best friend (who I dumped obvs).
I am now 46. My ExH will be 50 next year, and he is STILL doing the same old shit. He had a serious GF after me. He cheated on her too. Was out all the time. She left him eventually. So, here he is, almost 50, and still chasing skirt every fucking weekend.
My point is, that your Partner isn't going to change. So get out whilst you're young.
I have been wondering why your guys was in bed until 5pm today. Not a hangover if he wasn't drinking. All I can think of, is that he was with OW and shagging all night. What else could it be?
Fwiw, leaving ExH was the best thing I could have done. Like you, my first H was the only guy I'd been with, so I had nothing to compare to. I am now happily remarried and DH is the polar opposite of ExH. Hardly ever goes out, if he does, he's home when he says he will be. No lying. No cheating. Would much rather be home with me, than out galavanting. And the irony? He's about 10 times more handsome than ExH. My ExH was thin and weedy and short. My DH is tall, broad shouldered, dark, ridiculously handsome
You too can find Mr Right. But only if you ditch this sorry excuse for a man. Don't wait too long like I did. It drove me to a breakdown. There are lovely men out there, like my DH, that won't treat you like this. And when you find that, well, life is just amazing and you truly wondered why and how, you lived with such shit for so long. You'll have your lmoment in your next relationship, where you think "Ah this is how it's meant to be" x