Op was your relationship really that great before the split? I was a little sad in the beginning but quickly relieved I no longer had to put up with him haha. Eventually you may feel grateful to the ap for taking him off your hands, I sure to do my exs ap!

Stop blaming yourself. Your ex cheated because he has a character flaw, not because of anything you did or didn’t do. Time will heal your wounds. Practice self care right now.