Answer:
I am a word. I become longer when the third letter is removed.
You use a knife to slice my head and weep beside me when I am dead.
The more holes you cover the lower I go.
I fly when I'm on and float coming off.
I am a portal to another world which you cannot enter. Only you can see me but I can't see you.
No matter how little or how much you use me, you change me every month.