I just wanted to say you are doing great! Down days are part and parcel, let yourself feel them and deal with them however you need.
My ex husband did the same, 4 weeks after our son turned 1 he left saying he wasnt happy and didnt want to be married with 2 kids anymore. His parents said he was confused and to give him space, I told them to tell him to grow up and be a father! Turned out he was seeing his best friends sil who was about 18!
Long story short, that went sour very quickly after it all came out, he jumped into another relationship from which he had another child. I filed for divorce after my confronting him about the impending new arrival to be told that "it's not what he wanted".
It was a long hard road for about a year or so, I hated letting the kids go to their dads and interact with the gf, hated hearing her voice saying goodnight to them when he would call to day goodnight. Eventually though, I began to enjoy my eow alone, lie ins, long hot bubble baths with a good book, eating and drinking what I wanted. I eventually even started dating, but I kept my life with the kids separate. He left the mother of his other child after a year or so and has had relationship after relationship since! Hes just got engaged on new years eve, I hope hes finally happy.
Me, I met someone who is 11 years younger than me where I used to work, he seemed nice and had a cheeky personality but was nothing like what I thought was my type. He asked me out and I went with an open mind. We have now been together almost 6 years, married in 2018 and are 13 weeks away from the birth of our son. My kids adore him, he has been an amazing step dad and loves them like they're his own. I couldnt imagine my life without him now.
It might not seem like it now, I know I never thought I'd do all this again, but you will have a great life without him, and your kids have an amazing role model to look up to in you by being how strong you are and being their constant love and support, you are showing your kids what not to accept in their future relationships and they will be amazing grownups all because of you, never forget that!