It's normal to get jealous that he was once happy with someone else besides you; but it's over now, he's with you and loves you, not her. You just need more reassurance within your relationship and more confidence.
Just remember that everyone has an ex. I know it's hard especially if you have met them. Always keep in mind that he has asked you to spend forever with him. Look forward to a great future and let the past go.
Never compare yourself to others ( Galatians 6:4). It is not useful to look at somebody else and then look at yourself and try to figure out what they have that you don't. Being anxious about things like this can waste your life and energy. The only person you have to be better than is the person you were yesterday, Learn to love yourself for just being you and this will help you to allow him to love him. Sometimes we create problems in our heads that ruins the reality of the joy we have in our lives.