Question: I am a Trinity grade 5 pianist and have been playing piano for 2-3 years(earlier played keyboard).

I find very difficult to sight read. I usually play the passage once very carefully and memorize it well so I don't have look at the music while playing. I play while continuously looking down at the keys and find it difficult to lift my head up and register the notes and then play. I know all the notes on the staff, but I can't register them in time to play them on the spot.

What should I do to get better at sight reading?

Answer: 
As with anything, it will improve with practice. And, as with anything, *you should tailor your practice to the problem you're having*.

Since your problem is looking down at the keys, devise a practice method that prevents you from doing so. My teacher growing up always held a notebook above my hands so that I couldn't look at them. Do you have someone in your household or school that would be willing to do that for the 10 minutes a day you practice your sightreading? If not, you could always try rigging something to blog your view. (Or you could really be ridiculous and get one of [these](https://rads.stackoverflow.com/amzn/click/B076VYRT1B).)

Once you've mastered this, you may eventually have a problem with not looking ahead while you sightread; to address this, have someone cover up the current chord you're on, which forces you to look ahead. And so on.

See also recent responses to [What is a good way to practice piano sight reading?](https://music.stackexchange.com/questions/65879/what-is-a-good-way-to-practice-piano-sight-reading)
I'm going to approach this answer from a different perspective, my own. For years I had trouble sight reading because it took more time to focus on the notes and be certain of their place on the stave. I have problems with my sight where I see double images when I look at things from certain angles so I can see double notes, double staves etc. when trying to sight read. Very confusing when sight reading. I had my eyes checked by several doctors with no solution, but I kept trying and finally found a doctor who knew the solution. He tested me, gave me a new prescription for glasses and now I don't see double when I'm wearing them. If your problem stems from eye problems, you'll need to get that fixed before you do anything else.