" what am i doing wrong??!?!?!?"
"try to keep the heat down sometimes blowing with compressed air--"
heating things up and cooling them down with compressed air is called 'shrinking' and will warp the panel.
the new panel should just lay in place, if you need to clamp it down into place it will warp.
practice your welder settings on identical metal and try for enough penetration, not too much build up and the smallest 'heat ring' you can make, then use the 'tack and move' method, letting everything cool naturally.
The time you waste waiting is more than saved on finishing.