Hi Paul, and welcome.
Ideally, we should be only eating enough carbs in a meal to send the blood sugar up by 2 mmol/l, or less, after two hours.
So if you start at 5.7, you should try and not go over 7.7 two hours after eating. Likewise, if you start at 8, an acceptable rise is to go up to 10.
There are NHS guidelines telling us what our ideal levels should be, but to start with, keeping to the 'no more than 2' rule is a great start.
I'm guessing that the reason you felt so bad at 4.5 three hours after food is not because your blood glucose was particularly low. Instead, I think it was because it rose quickly and then dropped quickly. Rapid rises and falls like that can make you feel dreadful. Keeping to gentle rises and falls is much more comfortable.
The answer is to try and eat fewer carbs (potato, pasta, rice, bread, sugar, cake, biscuits...). The smaller the portion, the smaller the rise. And the better you will feel.
Why not try a few meals where you eat half portions of carbs (fill up on meat and veg instead, so you don't go hungry). And measure your blood glucose before and 2 hours after. That should give you a start to understand how your body is reacting to the carbs, and what size portions work for you.
Hope that made sense!
