Let's say that someone ch''v suffers from a severe psychiatric disorder, which can alter judgment and reality testing, e. g. Schizoaffective disorder (elements of schizophrenia and bi-polar disorder) and has been told by his rav, as this is the case frequently that his only mitzva is 'to get better'. What is the sense of this expression 'to get better'?
Very important question.
First ShizoAffective disorder is not a continuous and irreversible problem of Shote (see DSM 5), almost all mental illnesses has full remission states, or partial remission. Maybe that in certain acute situation, little quantities of time, he is Shote. Second. The answer, your only mitsva is to get better needs to be translated. The first of all step is to treat yourself in way to get a maximum of independence and mastery, a minimum of invalidating symptoms. After this step you can and you need to make all mitsvot. The man and the illness have to be considered as two entities. The condition for mitsvot is free will. one man who is in a very acute psychotic & / manic / depressive state loses his free will (see gemara Ketubbot 20a). Pharmacological, psychotherapeutic and rehabilitating treatments help to regain free will. So, in a state in which the amount of free will is low (See for instance Kiddushin 40a an exemple of loss of free will concerning particular things, according to some Rishonim), often, the only free will is to make all to get better, to make a great effort to get the better treatment.