My penis won't get hard enough to have intercourse with my Wife

I am 28 years old and got married 2 months back,i am Muslim before the marriage i was virgin i never had sex before in my life.

After the marriage when the time comes to start doing sex my penis rock get back to normal i tried many times to penetrate her but not successful at all times,i use tablet to make my penis rock hard enough to penetrate her,it over come this problem but still i couldn't finish the task,so my main concern is that why normally my penis rock don't gets hard enough to finish this job?

i did everything i didn't eat much in dinner,i relaxed my self,i kissed her that time it gets hard but after some time it goes back to normal,after that i have tried several time but nothing.

when i use the tablet even then i was not successful as my penis doesn't go inside.

Please advise