Have you tried stretching? Maybe some banded mobilizations as well? Does nothing seem to help you regain PAIN FREE wrist extension? If that's the case, try THIS wrist mobilization technique I learned from Dr. Joe Godges, DPT, MA, OCS while I was in China.
Place your hand on a table and slowly lean forward to increase the amount of wrist extension. Find the particular point where your pain begins, then STOP and come off of it a little. This is going to be your pain-free ROM starting point.
Using your opposite hand, grasp your wrist as close to the joint line as possible. Apply a constant pull across your wrist (medial mobilization). While maintaining that constant pull, slowly lean forward and increase the amount of wrist extension. If it's now pain free, great! Mobilize the wrist by rocking back and forth about 15-20x. This motion must remain PAIN FREE.
If your pain is still there or becomes worse - STOP. Change the angle of your pull by bringing your pulling elbow either closer or further away from your belly button. You may also have to adjust how much you're pulling by either increasing or decreasing the force of the pull. Now, while maintaining a constant pull, slowly lean forward and increase the amount of wrist extension. If it's still painful, adjust again. The goal is to find a specific mobilization angle and force that will allow you to move into PAIN FREE wrist extension.