Thanks for your query
Usage of high doses of steroids
can cause weakening of bones and decrease in bone density
This is a side effect of the drug and you have used it for quite a long time.
To recover from this you need to boost up your diet with food rich in calcium like milk,fish etc.
Also you need to get prescribed tablets of calcium and vitamin D to help you recover from the deficiency.
Slowly but surely you will recover ,keep follow up with your doctor.