Must I tell my future spouse about my illness?

Yes, an illness or defect that affects the marriage must be disclosed; concealing it is deception.
The Prophet ﷺ said that whoever deceives is not one of us. The Shafi'i jurists list defects such as insanity, leprosy, leucoderma and genital defects as grounds for judicial dissolution (fasakh) if concealed, and Islamic family law in Malaysia also recognises certain illnesses as grounds for fasakh. Other serious illnesses — contagious ones, those affecting fertility, or significant mental disorders — should also be disclosed out of trustworthiness. Minor illnesses that do not affect the household need not be made known. See Talaq & divorce on fasakh.