Hey there Got a quick question & problem:

What helps more in sight reading, writing your stuff or learning other people's stuff?

So, my sight reading has fallen back A LOT and well I want to get really good at it, better than I were, because I ain't definitely stopping again. The thing is that I'm starting from 0 again and I'm having a small problem: I end up learning them, thus not needing to read and I can't progress yet to other parts because I haven't reached that level again.

The only way to improve your sight reading is to sight read, which is not the same thing as learning songs. Sight reading is reading down a piece that you have never seen before, not working through a score you've been working on for a while.
Quote by poisonousmetal
The thing is that on two or three plays I've memorized it. So you'd recommend just advancing?

yes. at that point you're not sight-reading, you're just cleaning up the mistakes.

to practice sight-reading, never sight read the same music twice in the same day, and never do it more than two days in a row.
Anfangen ist leicht, Beharren eine Kunst.
Find a book or a website with a large number of simple melodies (Hanon exercises are great) in different keys, and don't spend too much time on any particular one of them. Set a metronome at a comfortable pace (slow enough that you can read the melody, but fast enough that you're still pushing yourself) and give each melody two or three attempts (if it's really difficult you can do a couple more, but no more than five). Just keep flipping through new material, never spending enough time on any one piece to allow yourself to memorize it. Memorization is not sight reading, and simply wastes your time.

Don't forget to read in more than one position, as well. If a melody is easy in first position, perhaps it will be challenging in fifth or eighth position. Transpose up an octave if necessary.