I recommend you to practise sightreading separately, for it takes your full attention. You might for instance practice pieces for 30 min. and then sightreading for 30 min. While sightreading make sure to look mainly at the notes, not at the keyboard. Avoid switching your view all the time, wich is a very common mistake among beginners. And learn the places of all the notes on the staff by heart. Good sightreaders link the notes directly with their respective key, without counting from an other note or reading from one note to the other in a relative manner.