Oh dear! A reading of 24 is not good and it must be making you feel awful, but unfortunately your choice of food wasn't helpful, as has been mentioned my
@janw. If you modify your diet, those levels should come down quite a bit and you will feel a lot better. You might not even realise how bad it is making you feel until your levels come down into the normal range and then you might feel rough for a while at normal levels until your body adjusts
Sweet potatoes and ordinary potatoes are both about the same carbs wise, so you are not really benefiting by swapping one for the other except that I believe sweet potato is higher in fibre and vitamin C. Celeriac is a good lower carb choice and is nice roasted or you can mash it or make chips with it and cauliflower is lovely mashed with a good dollop of cream cheese and a teaspoon of wholegrain mustard and topped with some grated cheese of course (pretty much any food is improved with the addition of cheese in my book!).... instead of mashed potato. You can also grate cauliflower and use it instead of rice with currys or chilli. I have a really nice recipe for halloumi and cauliflower curry, but I just eat it on it's own or I have Konjac rice with it which is pretty much carb free.
You need to learn which foods are high carbs and find alternatives to those when you are planning meals. Look at the "Nutrional information" labels on food. It will usually be in tiny print on the back or side of packaging. Ignore the traffic light system on the front. You need the information on "total carb content". If it is fruit and veg, then google will give you an idea of the carb content.
It is usually expressed as grams per 100grams, so you can compare individual items like two different brands of baked beans but then you have to consider your portion size. So if something has 60g carbs per 100g and you have a 50g portion, then you are eating 30g carbs. You need to tot up the carbohydrate content for each food in a meal to get the total..... so carbs in the potato and carbs in the baked beans with the meal you describe.... cheese has a negligible amount of carbs so you can ignore that. Test before eating and then 2 hours after to see how much your BG levels increased due to that meal. Keep a food diary with the readings and if your levels increased by more than 3 whole mmols then you need to reduce the portion size of carbs next time you have that meal.
I know it sounds pretty complicated but it really does work and you soon get into a routine with it that you no longer need to read labels because you know them off by heart and you learn which foods to buy and how best to cook them and avoid buying the high carb foods which make your BG levels high or just stick to very small portions of them if you are able. I am a bit of an "all or nothing" girl, so cutting out bread and spuds and rice and pasta was the easiest way for me. I get to eat lots of lovely tasty food still. It is just different to the food I used to eat.