Now a days my penis glans doesn't get big during erection, it remains almost same. Even my erection isn't so hard & doesn't last long. What is the reason?

How old are you? If you're in your late twenties or older, this is to be expected. Things will settle down but you're still a man. If you're younger than this, and you don't get super-excited even with your favorite partner / favorite images, get checked out by your physician, including checks for prolactinoma and hemochromatosis.