Traumatized children bathe in negativity and think the worst. That is a worldview shaped by trauma and hardwired into their forming emotional brain systems in the early years. Then, in order to make sense of things, they believe they are protecting themselves from the worst, by thinking the worst, before the worst happens. It makes perfect sense, even if misguided.
If you understand this, YOU can be prepared for the negativity when it shows up, so you can avoid being dismissive, discounting and disparaging. Those reactions on your part to their predictable negativity will prove to your child that their thinking is correct–the world is mean and unsafe. See how that works?
I know it seems like your child at some point SHOULD be developing a sense of gratitude for what YOU have done. You might think it will eventually dawn on them that they are living a fabulous, abundant life. And then it doesn’t.
Take heart. It will, but YOU have to provide safety and positive experiences to develop your child’s brain forward. In the face of negativity, show acceptance of the fear of the worst and with empathy encourage practice, accept missteps, and assure your child that you will be there to support them no matter what. Do this a zillion times and one day you will see joy and gratitude creep right in.