You are sick of not having control of your life, you constantly struggle with these insecurities and because you can't overcome them your mind instead of having pleasurable and stress free life with security in your relationship it adapted to survive with the opposite.

You relinquished control to your partner to have as many lovers as he wants so you don't have to worry anymore, after all he does it with your permission.

Maybe instead of worrying about being inadequate
for him you will also get into humiliation so you wont have to constantly worry about it.

Take the heterosexual male cuckold with small dick insecurities, he get off to drinking the cum of his wife's lover while he sees her get pounded by 4 people with big dicks this allows him the control and safety he needs in his life, his brain got wired this way.

By making the decision himself he retains the power of his ego, control over his life. It's simply easier to give in and let it happen or in this case make it happen.

Take the attachment types in humans like Anxious-avoidant and dismissive-avoidant where they wanted relationships with other people but because of fear of emotional pain they took their choice in there own hands.

Another example are rape survivors who may develop a rape kink but with their SO's or even deny the fact they were raped. It is simply the need for control because others took it from you.

Think the book 1984 although not the same I believe its very similar:

“He gazed up at the enormous face. Forty years it had taken him to learn what kind of smile was hidden beneath the dark mustache. O cruel, needless misunderstanding! O stubborn, self-willed exile from the loving breast! Two gin-scented tears trickled down the sides of his nose. But it was all right, everything was all right, the struggle was finished. He had won the victory over himself. He loved Big Brother.”

― George Orwell, 1984

If you don't wanna be a cuckold then you will have to completely overcome these insecurities and try not to feed them anymore.