guidance

Context: I'm 28, my girlfriend is 23. We've been together 5 years and I'm the only guy she's been with sexually. We're in the UK. I first brought up the idea of involving another guy in our sex life about 6 months ago with talk of a threesome. Not going to lie, I was hoping that this would lead...

My dick is really small, I think I'm only an inch and a half long. My wife has a really big ass and literally I have trouble getting past it to try to have sex with her (doggy) and I always slip out just due to my small size. It really is over for guys like us I think. Any other guys?