Replaceable

Each time you end, I always have another to replace you again.

Waiting for me to respond, as I get ready to refill; replace that cartridge for another.

Although you aren't easy to buy, I always find time;

time to replace you, once you've ended your stay.

Replaceable, that is what you are.

Easy to release. 

Easy to let go.

I have no worries because I can,

replace you again.