Hi,
@Shezzyk7 , welcome to the club!
There's a sub-forum thread to do with managing exercise and insulin which might be worth a look at:
https://www.diabetes.co.uk/forum/threads/managing-exercise-and-insulin.127691/page-8
The main thing to think about is that insulin lets glucose into cells to be used as energy, and when you exercise (and that is not only gym stuff, it's also just walking) some chemistry called Glut4, glucose transporter 4, comes to the surface of each muscle cell when they contract during exercise and those let glucose into the cell as well.
That can end up as a double whammy, both insulin and glut4 lowering bg, leading to messy hypos.
Doesn't mean you can't exercise, far from it, it means that when you're sitting down for a meal, remember that insulin lasts for about 5 hrs, so if you are planning on exercise during that time frame, think about tailing back the dose a bit, otherwise the insulin and glut4 together can get messy - you can tail back the insulin because you know that the glut4 will lower without it.
Knowing how much and when to tail it back is a highly individual experience and it'll take you a bit of time to learn what works for you.
It's not a precise science, because biology is messy, and you will make mistakes, we all do, but you can learn a lot from them, so don't get too disheartened when it's not playing by the rules.
The fixed 4u regime you're on is fairly standard for newly dx'd. It lets the docs get a better idea of ratios for you, but after you've settled down a bit, it'll be pretty much free range on dose amounts.
You'll end up making decisions about whether you need 4 or 8 or 12 units for whatever meal you want to eat so there'll be a whole lot more leeway on meal choices but in these early days it does make sense to rein things in a bit to reduce the amount of variables involved.
You'll be surprised by how much leeway there is. This site is mainly for T2 low carbing diabetics. That is a totally different condition with different management techniques, tending towards low carbing. It makes sense for them (T2s are generally insulin resistant, so carbs are always going to be a problem as insulin doesn't do what it's supposed to do).
With T1, though, insulin resistance isn't an issue - we inject insulin, it lowers our bg. With us, it's more a case of moderating portion sizes, thinking about matching the time pattern of insulin to the absorption rate of carbs.
I'd ignore any advice you get to reduce carbs - they are perfectly manageable once you understand how insulin works. That's not to say you should be scoofing full sugar gatorade and doughnuts - those would be difficult to manage. It's more about portion size.
You'll likely read about the wonders of low carb/keto. There's a place for them, no doubt about it, but they involve more or less ruling out an entire food group, massively reducing your menu choices when eating out, so don't make any decisions about those till you know a lot more - many of us find that we can manage and enjoy a reasonable amount of carbs if we pay attention to pre-bolusing (timing the injection before eating so the insulin has enough time to start working before carbs start turning into glucose).
Your docs running you at 11 to 19 is fine for the moment. In the long term, the aim is for the 4 to 7 range, but for now, it's a bit like bringing a deep sea diver up from depth: too quick and it gets messy, so do it in stages, as your body will have become accustomed to higher levels pre-dx and dragging it down to 5 straight away won't be nice.
It's interesting to see you've been given libre so soon. It's a marvellous device. Many of us are using it with 3rd party transmitters like blucon and miaomiao to turn it into proper cgm, so maybe explore that at some point. It's hugely reassuring being able to play about with doses knowing your phone will ring to warn you about a developing low.
I'd recommend some reading, all on kindle, all written by T1s who also happen to be endos or T1 hcps:
Think Like a Pancreas, Gary Scheiner.
Sugar Surfing, Stephen Ponder.
Beyond Fingersticks, William Lee Dubois.
Good luck, mate, I was an angry young man (21) in the first few months after my dx, but a year later, I was standing on a ski slope on a beautiful sunny day in the Scottish Highlands, I'd figured out enough by then to stitch T1 into skiing for 6 hrs and a mid-session bacon roll and it was all good.