Yes, their goals are different than our goals--the short term disability caseworkers are there to find a reason not to grant paid time off to you, but it sounds like you have a good therapist and a pdoc, so getting back-up from them shouldn't be too hard. You really are going through the worst of it right now. If you can get through this part you'll see it start to ease in time, I promise.

Can you ask for help with this from your therapist and doctor? Maybe a letter or something stating you absolutely do need this extension?

Good luck to you! Try not to take the caseworker's personality personally (easier said than done!) Honestly they act that way to everybody, whether it's PTSD or a torn ligament, they don't care. Hang in there!