Take heart! What you're trying to learn is difficult, and takes a lot of time and practice.
One particular difficulty is in finding your place in the score again after you've looked down at your hands. (Note that every pianist looks down at their hands at some stage in learning a piece - the effortlessness you see in the concert hall hides the hundreds of hours of practice which created it). Pay a lot of attention to bar-lines; be aware of which bar you "are" in, in which system on the page. (A "system" is a number of staves linked together because they're played simultaneously. For an orchestral score it might be up to 50 staves - for piano music it's almost always 2. Piano sheet music tends to have about 4-6 "systems" per page).
So think to yourself "second system, third bar", for example. When this becomes easier, become aware of where you "are" within the bar. First beat, second beat? Halfway through?
If this sounds like thinking about too many things at once (over and above playing!), that's because that's exactly what it is. It's hard.
The ultimate way to sightread is to aggregate. If this is your native language, you won't be reading every letter or even every word in this sentence. You might take in one of these short paragraphs "in one go".
Music is exactly the same. At the moment I'm learning Polish, which puts me in a similar position to you. At first what I read is just a mass of random letters (with far too many consonants!). Now I'm beginning to see not letters but sounds, syllables and words. If I get (much) better at it, I'll start to see phrases; sentences; ideas and arguments spanning many sentences.
In language and in music, the link between symbol, sound and meaning (what does "na" do in this Polish sentence? What does the note G do in this context?) is crucial. By learning how to sight-read more quickly, that is effectively what you're trying to learn. Just as I have to mouth Polish words (under my breath if necessary), you have to play what's on the page to learn how to read it. Which is tough, because you're learning how to play at the same time. When you get very good at it you can sightread music, and even start learning a piece, without a piano or even without moving your fingers. But before then, the link between reading, playing, hearing what you play and knowing that you've got it right (because it sounds right) is vitally important.
However good anyone's sight-reading is, it can be pushed back to close to zero when you encounter a different "language". Just like me with Polish. My teacher once gave me a Bartók piece to learn: I had to read it note by note. But, oddly, after a few weeks I could to some extent "speak Bartók", and tell when I'd made a mistake.
To make progress, here are some tips:
- Choose your music carefully. It has to be music you can rely on to clearly tell you when you get something wrong - by sounding clearly wrong. Beginner classical music is very good for this. Pop/theme-tune arrangements can be very difficult. They can sound awful, because they're badly transcribed for piano, or because, for them to sound good, the pianist has to "vamp" or improvise rather than playing exactly what's written (which is a whole other can of worms!). Also, this kind of music, though familiar to the ear, can be very complex musically.
- Little and often. Practice sight-reading on small pieces well below your playing ability. It's much better to sight-read 4 simple bars every day, than to struggle with 20 complex bars once a week.
In the UK exams are set by the Associated Board (ABRSM). Their sight-reading tests, at each level, are way below the difficulty level of the exam pieces themselves. That's a clue to how difficult sight-reading is. ABRSM publish books of specimen sight-reading tests, at various levels. They are great for practising: they can be just 4 simple bars, but there are a lot of them in each book.