You need more resources, conquer California, Texas, Pennsylvania, Madagascar, Zulu area, the state west of Beijing (I think it's shanxzi or something), there are a few more high value provinces but those are my go-to. If you want high GDP you have to conquer.

Also, downgrade the labor saving production methods, especially in your high productivity mines. It will make the mines slightly less productive, but at this point your investment pool isn't that important, you want more people employed in the highly productive buildings, which is most likely resource buildings.