I'm surprised you were told to eat cake/chocolate in moderation as I was told absolutely no way.
Some people don't tolerate some foods as well as others. What else did you have with the jelly? Was the angel delight the sugar free version? Perhaps you could try making it with full fat milk as fats help the carb release.
Also, did you wash your hands before testing? Sometimes things like hand cream can interfere with results.
I ate a lot of cheese. Full fat mayo. Protein bars by happy valley (I think, something like that).
You are supposed to have a small amount of carbs otherwise you end up with ketones which are not good for the baby. (Not sure why but when my ketone readings were high I had to go into hospital for a few days.) if you eat fats with carbs then this helps break the carbs down and stops you from getting a spike in your readings. So I might have a few cheesy chips with loads of full fat mayo and my readings were better than if I just had chips on their own. After following SW for years it was a treat to scoff cheese and full fat mayo.
Things like nuts are good to eat, full fat cheese, meat, veg.
Avoid fruit, sugary drinks, sweets, chocolate, mash (as it was already broken down it makes your blood sugar higher than if you ate a boiled potato).
I also went out for a walk after eating as this helped my readings stay low.
Don't skip meals. Don't go hungry as this could cause your fasting levels to rise. My morning readings were always high unless I had a midnight snack.
I'm not going to sugar coat it (sorry, can't have sugar anyway 
) but it was utterly miserable to begin with. After a few weeks I got to learn what food I could and couldn't eat and towards the end of my pregnancy it was fine. Not great but fine.
I know it's unfair and with Easter a few days away you want to treat yourself but this is about your baby now. I made a list of things I wanted bringing to the hospital once I had the baby and that kept me focused (and more than made up for it since).
There is a really good website called gestationaldiabetes.co.uk. It's got lots of recipes and the FB group is useful too.