Need help with sagging desk

Building this floating desk for my office. The front of the desk is sagging about a half inch in order to be level. Any ideas on how I could fox this and keep the floating desk style? Or is that not possible with my current build?

Table top is 5/8 pressed wood from Home Depot

Used 2x4 studs for framing

The frame is 13 inches from the wall. Held by 3 1/2 deck screws that go into the studs.

Is my issue that the frame is not really long enough to support this 24 inch tabletop?

Reason I built it small was so that the frame would not be seen from the front of the desk.

Any suggestions appreciated! Thank you