Any advice for well endowed guy?

I've been with my gf for a few years. We hardly have sex anymore. It used to be a lot but wound down to a few times a month. If I don't "finish" soon then she feels insecure. If I finish too quick then she is frustrated. I have really good control and can get an erection and stay hard as many times or long as need be, but she's only been able to climax once with me inside. Usually she doesn't let me go all the way in and if I do shes n pain so she usually pushes her hand against my stomach during sex. I'm only 10 inches hard and 8 around. Does anyone have any advice? Please don't reply with stupid comments or jokes. Serious replies only pls.