Metformin doesn't act directly on your blood glucose levels; instead it reduces the insulin resistance in your muscle cells so that the insulin you are producing can act more effectively. It helps prevent spikes, and it can aid weight loss (if you need to lose any). Usually it doesn't reduce BG levels by more than 1 or 2 points.
Glucose is produced in your body every time you eat carbohydrate; insulin from your pancreas puts the glucose into your muscle cells (where it's used for energy) and stores any surplus in your fat cells. Your liver stores some too as a reserve. If your muscle cells 'resist' taking in the glucose, it gets converted into fat and stored in your fat cells.
If you reduce your carbohydrate intake, to about 150g or less, your blood glucose levels will drop. Many of the Type 2s on here eat a reduced carbohydrate diet and find it works well. Some Type 1s eat this way too.
Re the Metformin: don't go straight up from 1 tablet a day to 4 - it could cause horrible gastric problems :shock: . It's more usual to take 1 a day for a week, then 2 a day for a week, then 3, then 4. This allows your system to acclimatise.
Hope this helps.
Viv 8)
PS I don't quite understand the levels you give - were they from an HbA1c? and do you mean 5.8 mmol/l (which is a good reading), or 58%, which is high. V but see Sid Bonkers' post below.