Like previous responses, I've travelled a lot by air over the last 40 years or so, both longhaul and short haul, and have never had an issue. Whilst I do carry a doctors lettter, I have never in all those years been asked for it. I still carry it as an "insurance" with my other flight papers, even though it's quite aged now !
Years ago, I used to regularly mention it at security, but never a problem, and certainly for some years now I don't bother. Most important I find, is to make sure all your diabetic stuff is in your hand luggage, and insulin is in a cool bag. As previously mentioned, always take some extra supplies as I find, differing temperatures abroad can make quite a difference to insulin requirements, but you can deal with this by way of adjustments, or temporary changes. One never knows either about flight delays. I was abroad when covid kicked off and return was delayed by quite some time. All dealt with ok because of extra supplies.
Like you, I use a Libre and have never had an issue going through security.
I suspect nowadays, type 1 diabetics at/on airports/flights are so not unusual, and personally, I have never felt the need, certainly over the last ten years or so, to even mention it let alone declare it as a disability.
Have a great time, and do not be nervous. Nothing at all to be nervous about........enjoy !
Once you've flown, I'm sure you'll wonder what you were ever concerned about. Just remember, you're in charge of all this and can deal with anything.