Will I be held accountable for the evil prayers I invoke upon myself due to severe obsessive-compulsive disorder that prevented me from performing acts of worship, and am I a disbeliever because I missed some prayers in the past, and will Allah answer those prayers?

The answer is summarized in two points: First: The treatment of waswas (whispers/obsessive thoughts) has been explained in previous fatwas. Second: supplicating against oneself is forbidden and impermissible, as the Prophet (peace be upon him) forbade supplicating against oneself, one's children, servants, or wealth, because it might coincide with a time of answered prayers.

Your statement: "If I succumb to waswas in my prayer, for example, and do not perform it or leave before completing it because of waswas, then I am a disbeliever" does not make you a disbeliever, because your intention is to encourage worship, not to be content with disbelief. You must repent and seek forgiveness. There is no expiation required of you, according to the more sound of the two scholarly opinions.

As for your conditional vow to fast months or years if you succumb to waswas, you are not obligated to fulfill it, because it is a vow of contention (nadhr lajaj - a vow made in the heat of an argument or out of anger, essentially a form of oath), and you have the choice between fulfilling what you vowed or offering an expiation for an oath.