I'm worried about the fact that my ex was better in bed than my wife
Do you regret marrying your wife because of this?
I made a big mistake when I married her.
It's rude to compare your wife to your ex's sexual performance. If you have a problem with your wife you should talk to her about it.
I can't, she might think that I want to break up.
You shouldn't tell your wife you compare her to your ex, but it's good to be honest about things like this. It doesn't have to mean you want to break up, just that you want to make things better.