Fear
Fear is a consciousness, an awareness of reality. The concept that there is a reality; and that there's threats to your place in it.
Fear is the knowledge that reality can be taken away. That you have to cling onto what you've got, with everything you've got.
Jay knew a thing or two about fear.
He knew about awareness. He knew about threats. He'd seen them; fought them even. Battled tirelessly against them. Relentlessly and endlessly. Over and over.
Because he knew that there was no such thing as second chances. No fresh starts. No second go. Every second was individual and measured, unchangeable and unique.
That was his fear. To want to change time. Because he knew he couldn't.
No second chance.