What is the ruling on the Istikharah prayer for marriage in general, when marriage is obligatory or recommended, and the person fears falling short in marital duties, or the marriage is affected by a mental illness?

Seeking guidance (Istikhara) is recommended for permissible matters, and it is not performed for obligations, forbidden acts, or disliked acts. As for recommended acts, one does not seek guidance concerning their essence, but rather in choosing between two recommended acts or in their order. Therefore, if marriage is obligatory or recommended for you, do not seek guidance concerning its essence. However, if it is permissible, there is no objection to performing Istikhara regarding it.