How to overcome erection difficulties during sex that induce depression in sex life?

Hello doctor,

I am 37 years old. Before my present partner, I had never had a girlfriend. Even so, I had the typical heartbreak due to unfulfilled one-sided love, thoughts about having sex with ladies, etc. We have been together for four years and are pretty happy. I tried to have sex with her for four years but could never get close to her. I occasionally lose my erection or have no one. Even still, there are occasions when I have an erection but cannot get into her, and the erection goes away. She is only 25, and I am 36; she was sexually active before we met; she is frustrated and has even chosen to leave me. Her sex life has been shattered since she began an affair with me, she told me with glee. I am concerned. I even engaged in gay sex to determine my sexual orientation. I did not like either, but now I fantasize about having sex with both sexes even though I failed to enjoy either. I masturbate three to four times a day and watch much porn. Due to the absence of sex in the relationship, she is extremely depressed and frustrated. She loves me, but because we have not had sex, she is looking for intimacy elsewhere. I stumbled upon one of her conversations with one of her friends. She appears to be considering leaving and following her. Kindly help.