I feel insecure sometimes. I see that your biggest worry is that you will be replaced as a (sexual) partner by your gf. Ask yourself how likely that is. Is your husband with you just because he hasn't found anyone else, or are you special to him?
Sometimes you need to fake it to make it. Insecurity is like a constant tape playing out in our heads "I'm not fun/sexy/smart/pretty enough". Or even "If my partner truly knew what I was like, they wouldn't want me anymore". So instead of listening to this broken record, try switching your internal dialogue to something constructive instead. "My partner is with me because they value me as a person, not just because they can't find someone else. They have the option to walk away anytime, yet they choose to be with me instead."
Me: bi female in my twenties