Hello,
Desoximetasone is a topical corticosteroid and is not used for fungal infection. For nail fungal infection, topical and oral antifungals are needed which are available under prescription. Also nail fungal infection may take a bit longer to go away. So it is important to use the topical and oral anrifungals as and when prescribed by a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.