Yes, the sentence is grammatically correct. It means that the speaker finds the last phase of a situation very stressful and acknowledges that they cannot deny or pretend that it isn’t stressful.
It can be rephrased in several ways, such as:
1. "This final stage is extremely stressful, and I can't pretend otherwise."
2. "I’m finding this last phase to be very stressful, and I won’t deny it."
3. "I can’t deny that this last phase is