Upon review, I think the answer may be
I'm given once and only once,
We all only get one life
but you can take me for months and months.
You can take breaths all the time
You wish you were able to keep me for longer
Many of us wish we could breathe indefinitely
but try to hold me, I promise I'm stronger.
It's very difficult to hold one's breath for too long.
Also as CiaPan points out in the comments, once you stop breathing you soon lose your consciousness and then breathing resumes.
You can try to refuse me, to not let me in, but this one is deadly for you and your kin
Again, not breathing in can be deadly
Say "breath" and you make a breath.