Radiotherapy - deep inspiration breath hold

Hi hoping someone out there will have some experience of this.

I had a simulation today but didn’t know what to expect and thought all I needed to do was to hold my breath for 20 sec so was surprised to see just how high tech it all is.  I failed the simulation as I could not hold each breath consistently, I found the whole thing confusing and exhausting. I was told I have to try again in a weeks time - can anyone give me tips, tell me of their experience. This I’d delaying my radiotherapy.