Ah op it's really difficult and hard.
I think though you're giving this guy a lot of power over you. If you were able to step back a bit from the relationship and think objectively about what you actually want in a partner I'm guessing this guy wouldn't actually fit the bill.
I'm sure he has good traits or you wouldn't have been with him, but he's also shown you that he's a liar, he's sneaky, he's disloyal, he's not serious about future planning, he's selfish, he's narcissistic and puts his own needs ahead of others. My guess is the other woman he's involved with also doesn't know she's been sharing him for the last year. You had an idea of what the relationship was, and who he was and that was an easy pretence for him to keep because of the distance, but that was never the full picture.
You need to have a think about what it is you actually want from a partner, what type of relationship you want and what qualities you want in them and then focus on reminding yourself that is what you deserve. I know you weren't exclusive and there were no labels, but dating someone for a year and omitting that is poor form to be honest. I know technically he's not wrong, but it feels underhanded. I think part of your reaction is also in how this has made you feel and how its affected your self worth, so again you need to think about what boundaries you need in future relationships to protect you from that going forwards and what conversations you need to have to ensure you're on the same page. This guy really doesn't deserve this level of emotion from you, but it's understandable you're grieving who you thought him to be and what you thought was in your future.
I think often after an abusive relationship we can find ourselves in a relationship that feels healthy but actually isn't, it's just healthier than what you'd had previously but actually your standards need raised a bit higher. I think essentially you feel cheated and because of having no label, you have been unable to vocalise that leaving you kind of stuck with it. You can't do much with that but you can block and delete him, and give yourself freedom from him in order to move on.
Focus on doing what you have been, keeping busy, keeping routine, eating and sleeping well, stick it out with the counsellor, take the time and energy spent on him and invest it in yourself and other relationships with friends/ family / your son until you're ready to date again and remember time really is a great healer. Grief is something we learn to gradually accept in our lives and we grow around it and some days will be harder than others but with time those hard days do get a bit further apart. You're doing all the right things, you deserve good things in life and you deserved better than what you got here.