@mountaintom you'll regain the weight now that you've got insulin to help your body fully utilise the food you eat. But you've a great opportunity now to eat well to make them 2.5 healthy stones you add!
Go slowly with the exercise, your body has been and is still going through a big shock.
@NoKindOfSusie I think is right in that we're never going to be exactly the same as before overall. However, exercise should be a part of life where there should be little difference in your performance before and after, assuming you have no significant complications. (Acknowledging there is a lot more faff as you need your glucose and testing kits, to plan it in around meals / taking insulin).
My marathon PB before T1 was 3hr 38mins... post it is 3hrs 26mins. But I definitely could not have done that within early months of diagnosis, I couldn't run then either!
Or current England Rugby Union player Henry Slade, age 24 (diagnosed T1 at 18). He seems to still be able to run around for 80 mins... he also has the added problem of 120kg opposition players repeatedly try and smash him into the ground on his runs!
Point taken that you don't want to be an England Rugby player, but isn't he of a similar age to you now and was also diagnosed as an adult?
Exercise will help your mindset, but also improve your blood sugars and lower your insulin requirements. Win / Win / Win.
Putting aside diabetes, if you were previously active but then didn't do any exercise for a few months you'd be feeling gross regardless. And if you did try it it would be horrible as all your muscles would have atrophied, your cardio fitness would have long gone. That's also at work here.
Well done for trying to run. Maybe you tried to soon originally.
If you're going through hell, keep going!