Hello and welcome,
A level of 15 is too high and may cause other complications if it isn't brought under control, no matter what caused the diabetes in the first place. Of course, if you come off steroids things may improve on their own, but meantime I would concentrate on getting those levels down to at least single figures, less if you can. The NICE guidelines, used by the NHS, advise between 4 and 7 (fasting and pre-meal) and under 8.5 at 2 hours after eating. These levels are over-generous, and most advice now is under 6 fasting and pre-meal, under 7.8 at 2 hours post meal.
If I were you I would aim to get your numbers down to at least the NICE guideline levels to begin with, then you can try for lower if you wish.
Perhaps we can help with this? Ask any questions you like, and @disy1 will be along with some advice for new members.