The trick is catching yourself when you're feeling ungrateful because it can be subtle. You might be preoccupied with doubt or you might be nervous about a presentation. Either way, your mind is too busy to court gratitude.
When you realize your mind is in a frenzy or burdened with doubts, you can free yourself from the quicksand of negativity. All you have to do is read something uplifting or do something kind for someone else.
There's always a way out when you have low-grade gratitude - that is when you keep an eye on your mind.