Why no erection during sex but possible through rubbing or masturbation?

Sir I am getting normal morning erection but can't get erection when I see or think about sexual activities...getting erection when I rub my penis or while I mastrubate. This occurred recently when I got sudden numbness in penis and strech in lower back when I got up after sitting on ground and having left foot pain(not constant) . I feel something in my legs when I try to get erect. Sir the nerves of my penis get pulled and it gets numb sometimes and I feel anxiety in and get scared for things I didn't face any such problem before please can you tell me the cure sir Thank you soo much