If your condition is unbearable for you and the dermatologist has advised you that you can only get relief by temporarily taking off your hijab, then I think you fall in the category of those who are excused. In Islam there are exceptions allowed for people in similar cases. This can be proven by pointing to the concession the Prophet, peace be upon him, allowed some of his companions to wear silk because of their medical condition, even though wearing silk in general is haraam and therefore considered as sinful for men in ordinary circumstances. Likewise, if your medical condition requires that you should take off your hijab, you may do so until such time that your condition improves, without incurring sin.
But I must caution you against resorting to this action purely on your own personal likes or dislikes; it can only be done if there is a genuine medical reason. For exceptions in shari’ah are simply exceptions and not general rules. They are bound by specific conditions and circumstances in such a way that the allowance is lifted as soon as the condition or circumstance changes.