Hi I just started to learn sight reading on guitar , I was started on Key C and no problem because it is easy but when I play on others keys I am confused about method or system for thinking on sharp or flat keys and connect to fretboard forms. What metod or system you all session musician use in acareer or in a real world.
It just takes practice. You probably spent the most time in C so that is easiest. Play the next few weeks only in the key of G and see how that starts to feel then. My guess is it will be easier.
I also want to suggest that you think of sight reading as something you practice, as opposed to something that, at some point, you should just be able to do. I play for a lot of musicals and even though I "sight read" the scores I spend hours practicing them too, they are just not memorized so I "read" the score while playing, but have positions and changes marked etc..
I would not spend to much time looking for a "system" to make it easy. Real music is unpredictable and an "system" will just not work. You just need to have put in the hours of practice in order to be able to do what you want to, that is both the good and the bad news.