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: 
Would my approach be of any help to you?

I'm doing Grade 3 exam piano pieces but my sight reading is poor. I suppose it can be explained. I spent many hours practising and polishing pieces. Comparatively, I worked relatively little on sight reading. I'm now evening up the emphasis and I hope it will pay dividends.

To train, I think you have to avoid becoming familiar with the music. So I have bought lots of short exercises (over 100 in all) and I play them at random. If I am in danger of knowing the music to the point where I can play from memory, I stop playing it for a few weeks which usually does the trick. This is different from reading the notation as one would the text in a book. I really want to develop the skill where you read the notation and hear the music in your head without playing a note on the instrument. 

There are various method books, YouTube videos, phone apps and books of practice pieces at the right level out there. I had to experiment to find what worked for me. I have not found a magic method that doesn't require a lot of work. I'm on my third method. It has taken me a while to get here. I am now happy enough with the method that I decided to stop looking for a better (magic) method and just get on with it. 

I set myself an aim to practice for at least 20 mins every day. It takes a lot of mental effort, so I make sight reading practice the first exercise of the day. If it goes well I might do more; if it goes poorly I might go back to simpler exercises or stop after 10/15 mins if my head just won't take it in. I found sight reading exercises too hard to be tacked on to the end of practice after working on exam pieces and technical exercises. 

Hope this is helpful for you.
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.