I can only maintain an erection during oral sex

I find my girlfriend of seven months very attractive, so why do I lose my erection during intercourse?My girlfriend and I have been in a relationship for seven months, and we started being sexual four months ago. But I can’t get erect without oral sex and, during intercourse, I can’t stay hard for more than about 40 seconds. I find my girlfriend very attractive, and there are strong sensual feelings and love between us. Why is this happening? What is the reason for losing erection du