Porridge itself may be enough to raise your blood sugars too high because of the carbohydrates in it..... you may be best changing to a cooked breakfast like bacon, eggs, sausages, mushrooms, omelette. If you DO have porridge do you make it with water or milk? if you use milk try unsweetened soya milk instead.... or Almond milk (also unsweetened)..... pinhead oats or oatmeal may not be so bad for you as rolled oats.... they seem to release the carbs more slowly. SOME people can't seem to manage any amount of carbs especially for breakfast though their tolerance increases throughout the day.
I was only in the pre-diabetic range of diagnosis and diet change has led me to having a normal HbA1c..... Most of us would suggest a lower carbohydrate diet - some people like the book Carbs & Calories and I use The GI GL Counter by Wynnie Chan - the first of these two books is available in pocket size or a bigger version and has pictures of meals shown on a plate (it gives the plate size) so you don't need to weigh stuff out but I gave mine away and just use the other...... I bought mine off eBay for under a fiver.... I think I got the carbs and cals off Amazon through a DUK website link....
I guess I haven't really answered your question about sweetener but if I sweeten mine I use Jordan's Skinny syrups in lots of flavours - sold for drinks/cocktails but also nice in porridge (if you can tolerate it without your bs going too high) but I know that you would also have to buy that too....
Again, if you do have porridge make sure you weigh your oats out - I tend to make my own mix with nuts and seeds and a small amount of dried fruit (but best you don't add that yet) along with unsweetened bran sticks and some oats. My weighed amount is around 70-80 grams - but that is my own personal tolerance....
I know there's such a lot to take in to start with so hope I've not bombarded you too much.
Hope this helps .....
Jan