I love what Jill said "There is no way to apply current wisdom to former mistakes."
You did the best you could in your situation and you left. Feel proud of that. One step at a time mamma.
When you start to feel down, do something small to feel positive. Don't allow yourself to go there.
Joining a group - whether it be a gym (some offer daycare), or a mom group, or whatever it is - or just getting out at lunch with work friends for lunch - just being more social will really help. A support group is even better. Your ex wasn't company and he wasn't positive. He was just 'comfortable' but in a very destructive and negative way.
Being comfortable on your own is where you want to be, so that you are ok on your own and it will come. You need this step so that one day you will be confident on your own, and that's when you will meet someone new. Don't skip this step or rush it.
Whatever you do - don't rush into meeting someone new.
Take this time to enjoy it and socialize with female friends. If you haven't had time for any or weren't able to socialize, make some now. This is your time. Focus on your children. This is time to just take care of you and them.
You can do this - you should feel good about your decision. It will come - I have friends who have left abusive relationships. I left a really long relationship myself (before kids) - it wasn't easy but ultimately, I found happiness and self respect and it was the best thing I ever did. It felt very hard at the time but good things often come with hard work and dedication. It certainly won't be as hard as what you've already been through! Just think of it that way.
If you need encouragement just come on here! Lots of moms have been through it :)