I think it just takes time. Dating is great. You don't have to settle down with anyone until you're certain.

As a parent myself, it's best to not get involved in the children's lives until you're absolutely sure you're wanting to take this relationship seriously.

If you're still thinking fond memories with your ex as new experiences come to play, I suspect you're not all that interested in this new woman. And you know what? That's ok. Keep trying meeting new people.