I have been on nph and log insulin for 18 years , can i replace my insulin with doses of 500 or 1000 g of metformin only?

Ask ur Doctor: This is the kind of thing that can only be done under the supervision of a doctor. Your doctor will know best , there could be other reasons for your tiredness.

Doubtful: If you have type 1 dm, absolutely not. If you have type 2 dm, unlkely. Any trial of adding pill(s) and decreasing/stopping Insulin would need to be done under very close supervision of your doctor, with frequent visits to monitor your progress. Metformin is good medicine, but can only improve your body's response to insulin, not replace insulin. After 18 years, many type 2's make little insulin.

Unlikely: While it is possible, it's very unlikely. Your doctor can check if your body is still making your own Insulin (type 2 diabetes) or not. By the way, nph is an outdated insulin. Check with your doc about replacing it with gargine (lantus) which is a true once daily basal insulin.