I can't stop imagining my wife fucking me

Any time I watch porn I deliberately find women that look like her and imagine that it is her getting fucked by someone else. I've told her my fantasy and we have text and roleplayed about it but she won't go through with it. The closest time was when we sent a potential bull a pic of her riding her bbc dildo that was seconds old. she just couldn't pull the trigger on me inviting him.