Disputing Parking citation at Sabine park?

So yesterday I went for my evening run at Sabine park and as usual I paid for my parking via the park mobile app. When I returned from my run there was a $70 parking citation on my windshield. Supposedly there was a private event and I was not supposed to be parked there but there was no signage… I went to the courthouse today to dispute the citation but they said the citation is valid and there’s nothing I can do? What the actual f*ck?