Hey, absolutely nothing wrong in ending it. I would have ended it too.
What I’d like to tell you is, pls don’t let his choices, his life, his happiness etc impact you negatively. Focus on yourself, things that you can control and become happier. You need to heal, become a happy person.
In the meanwhile, your ex might marry this woman, they might have children, or she could turn out to be the best partner he could ever hope for. So what? You need to stay strong and focus on yourself. Think of how you/children can benefit from the situation. Ask ex and his GF to take to kids out and you could use the time to relax.
As others on this thread pointed out, try to be a good mum to your kids (I’m not saying you are not - I don’t have enough info to conclude either way, so just putting it out there). I know it’s human to have good days and bad days, but don’t forget to tell your kids EVERYDAY how much you love them. Give them hugs and kisses often. Give them small things wrapped as presents (a chocolate bar, small toy, a ping pong ball etc). They will fondly remember getting these little surprise presents in childhood. Making good memories is cheap. Go for it !!
Do this for a few months, you won’t feel threatened by his GF or anyone!