I was trying to tighten my front shifter (the cable felt loose) and now I regret I did it. I messed it all up and it wouldn't shift into any gears. Then, I read how to use high and low front derailer screws to adjust gears. I got that to work, but now the shifter keeps downshifting by itself. It takes a lot of preassure to upshift, but almost none to downshift. So much that it downshifts by itself. How do I adjust it?