What should I do about my constant doubt concerning purity and the obsessive thoughts related to ablution (wudu') and ritual bathing (ghusl), which cause me to delay prayer?

What you are mentioning is a whispering from Satan (waswasah), and its cure is to turn away from it and not pay attention to it. Perform istinja' (washing after relieving oneself) normally by pouring water over the impure area without excessive effort until you are reasonably certain that the impurity has been removed. Then, perform ablution (wudu') normally by washing the limb once, twice, or thrice, by pouring water over it until the water covers it completely. Do not pay any attention to the devil's whisperings after that, for by doing so, Allah will remove this ailment from you.