Can you reach out to your diabetic team?
Everyone speaks well of the DAFNE course, but there is a fair waiting list. Ask your team to put you down for it.
In the meantime, BERTIE online may help you
https://www.bertieonline.org.uk/
I assume you're on a basal/bolus regime? The basal is meant to carry you through when you're not eating, so if you fast for eight hours (eg overnight) your blood sugar is the same before and after the fast. (That's also assuming there's no bolus insulin or food in your system at the beginning of the fast.) Once your basal is right then you can mess around with the fast acting insulin (bolus) to cope with your day time meals.
Your team should tell you how to calculate correction doses for insulin (eg for me 1 unit of insulin brings me down 2mmol/L but that number varies from person to person), and doses for carbohydrate (eg 1 unit of insulin for 10g of carbohydrate but again that figure varies widely from person to person).
Once you know your correction dose and carb ratio when you test your sugar before a meal you can add your correction dose (if your blood sugar is too high) and your meal amount (based on your ratio and the carb amount in the meal).
Once you know how to calculate correction doses you can at least pull your blood sugars down to normal if you find them too high.
I'm T1 so hopefully someone will correct if I've gotanuthing wrong for a T1.5.
Good luck.
ps my clinic was able to calculate my ratios pretty easily when I came in with a food and insulin and blood sugar diary.
pps am assuming you're in the UK. If you're not then ignore the DAFNE reference but you should still be able to do BERTIE.