"...when i changed my shifting cable it started to shift 2 gears instead of 1... might indicate a problem with the cable or installation. Did you use the correct type of cable? Did you prepare the cable housing ends correctly? https://www.sheldonbrown.com/cables.html#cutting Is the cable routed correctly, especially the final loop going to the derailleur? No kinks or sharp bends?
Edit: Is the cable head properly engaged in the shifter?