What Am I Riddles

Question: You struggle to regain me.
When I'm lost, you struggle to obtain me.
I pass no matter your will,
but I'm your slave to kill.
What am I?

Question: What is something that you always have with you but you always leave behind?

Question: A trail, a union, together tied.
Come across me and you will find,
you cannot change the course I'm on,
without me you cannot travel on.

What am I?

Question: I have no wings but I fly,
I have no teeth but I bite.

What am I?

Question: The more you work, the more I'll eat.You keep me full, I'll keep you neat. What am I?