Once you start going down the road of thinking about his ex (you know, that girl he dated for years before he met you?), it's hard to backpedal.
I mean, how can you, the new girl, even fathom it? He was in love with someone before you. Things between you two are so amazing that an idea like that doesn't even compute.
He was in love with someone else?
And then, after the nausea starts to take hold, the insecurities begin to creep up.
What if he liked her better than me?
What if she was better for him than I am?
What if she tries to get him back? What then?
These thoughts are toxic. And if you give them an inch, they will take over your mind and make you feel worried, anxious, insecure, and unworthy.
She's not better than you.
Get that through your mind. First things first: if he wanted to be with her, he wouldn't have broken up with her. And he wouldn't have started dating you.
Maybe he did have feelings for someone else before you came along. But didn't you, too? We meet and fall in love with so many people in this life. It's not a crime, on either of your parts, to have been with somebody else.
You are amazing. And beautiful. And unique and special and everything he wants. Stop worrying about that other girl; she's fading into the past now. Your insecurities and jealousies are all that's keeping her alive.