Concentrate on your breathing before. Sit and really watch your breathing and see how effortless it is. It just happens. Just observe this for as long as you need before you attempt your journey.

Imagine the breath coming into your body. Imagine it going out. Imagine you are breathing. Close your eyes and concentrate on this one thing as much as possible.  Feel the breath coming to you. Ignore the panic signals as much as possible. What you feel as an entity is your own panic.  You control what is within you.

Some itchiness on the back of the throat can be a bit irritating from DMT. Being thirty in hyperspace sucks too. I always eat fruit before and after a DMT trip.  I always hydrate and make sure that my throat is quenched and moist before vaping DMT.

Have everything you need right by your side. Have a bucket ready and retch if you need. Do what you need to as Tattvamasi stated. But try and concentrate on your breathing. Find your breath. Trust and know it is there and happening. 

Don't be in a hurry.  Some of what you are experiencing could be like running and being out of breath.  Slow down and walk. Breath. Pay attention.