Justme, he seems to be following the same script as my STBX H, he didn't love me anymore, he'd been distant and detached for months, completely denied any affair, made me feel it was my 'nagging' that that made him want to leave. He was having an affair. We'd been together 22 years, married for 16. MN people on this board were much wiser than me and helped me to realise the truth. I hacked into his secret email account and found the proof. Unknowingly, I did as MrsJA suggested, I offered to move out myself as I felt it was my fault. He really wasn't happy with that, it would have impacted terribly on his affair! Not saying you should try that, though, just in case! He wanted to leave me, make me feel it was my fault, then 'start' the new relationship. Keep it all amicable, keep the DC's respect, have it fucking all.
How's your sex life been? Ours had been non-existent for months. He was very possessive of his phone, had changed pins and passwords, had started sun-bathing and being concerned about his appearance. Does any of this sound familiar?
Be strong, you will be amazed how you will cope. Get RL support, get a solicitor. (((hugs)))