I feel so so sad for you, this is frighteningly similar to what happened to me, even down to the mental health issues, the coming back for chats, giving me just he tiniest scraps of possible hope that kept me in a hideous state of limbo and grief and craving him back. To cut a very long story short, it was another woman. Dug a little deeper and he'd been cheating right from the start with various women. He has now married the other woman and has even tried to get back in touch with me, moaning he's not happy. I could never have ever thought he was capable of at of it the lies, the deceit, the utter lack of care seeing me distraught and heartbroken. I relate to that feeling you have of something darker and off swirling below the surface when you see him.
Please don't be me. I was so in love with him I let him keep coming back and walk all over me for (I'm very ashamed to admit this) several years after he left the first time. If I could go back in time I would block and delete him in every aspect of my ife from day one, I would have healed so much faster. Definitely think about some fun dating, it was the only thing that worked for me. And guess what, as soon as he found out, he was round here like a shot, telling me what a mistake he'd made and how much he loved me and he needed to be with me...all lies of course.
Here for you lass should you ever need to talk, I know exactly what you are going through and it absolutely a feeling worse and more painful than grief, it really is horrible and I get how you want him back and for every thing to go back to normal but it never can now.
He has broken that trust on every level, you never really knew him if he was capable treating you like that. So many men like this, selfish, full of ego, just awful. Mine came across as the most caring, steadfast, amazing guy. All fake.
My best wishes to you at this very horrible time but I promise you, the sooner you can get him out of your life, the sooner you will heal. Stay strong, you are worth so much more than this.