Is it permissible for me to pray that I contract diabetes?

The basic principle is for a person to supplicate for themselves for goodness, well-being, and the completion of blessings in their religion, worldly life, body, wealth, and family; because a servant has not been given anything better than well-being. The Prophet forbade supplicating against oneself, one's children, servants, or wealth with illness or harm, out of fear that it might coincide with a time of answered supplication. So, whoever supplicates evil upon themselves has wronged themselves and deviated from the Prophet's guidance.