Things happen during manufacturing, which can make you lose your ever-loving mind, when sawing. One of these possibilities is that you JUST CANNOT saw a straight line or you are drifting to one side or the other. It might not be you!
Change your saw blade. If it keeps happening, open a new bundle and try again. If that doesn't work, use a different saw frame. If that still doesn't work, try a better quality of saw blades.