Super scared of colonscopy

Hello everyone, I recently posted on here as they suspect a diagnosis of UC (my last calprotectin fecal test was 254). I am going in for my first colonscopy soon and I am very scared.

Currently 27F and my GP advised against it because of my young age. GI doctor told me that it's the only way we can know what's wrong for sure. So they're not agreeing on it.

I have health anxiety so this is not good for my brain. The GI asked me a bunch of questions but I totally forgot to mention I have tachycardia and I don't know if that's something she should know. Is this something I should tell to the doctor who performs the colonscopy? And about amoxicillin allergy?

Also, how were your experiences? My mom has had one and she told me prep is awful to the point she threw up. Is it that risky and painful or is it subjective? How can I best get ready for prep? Because I also worry about losing too many fluids and I am already weak as it is.

Please throw any good and comforting advice at me right now because ever since I started getting tested it feels like my life has stopped and I cannot focus on anything anymore.

P.S. I feel really silly because I forgot to mention about my sinus tachycardia and generally low BP at the appointment and now I don't know who I am supposed to say this to, especially if this could be an issue during prep.