I measured the saddle height again, and came out to 73cm.

I actually think the stem is already flipped down, I attached a pic. Also looked on the site and the 56cm comes with a 100mm stem. Measured it again and it is 10cm. No clue how I got 11cm, all my measurements could be +/- 1cm.

If my core is what should hold me up, then I am definitely riding wrong. Need to just get stronger.

Thanks.

EDIT: Going to raise the saddle up 2cm and see how that feels. Waiting for it to get dark so I can go ride for a while. Very hot today.