Hi Frank.

I will show you on HTML/CSS view - maybe you will understand more how it works (Preview from Code Inspector/Firefox)

As you see there is only 15px left padding (violet color), that you can remove using Custom CSS:

#sp-header > .container {padding-left: 0;}

If you have bigger left padding (space) it can be also from your logo image, many times I have seen logo images that had huge padding inside image file itself. Then you have to cropp image using Photo Editor.

Left yellow collor is from container, becuase header is centered to be in the middle.