Flat Affect. "flat affect" is not something you diagnose someone with, but it's an observation that might go in one's mental status exam. Treatment depends on why the affect is "flat." it can be from depression, schizophrenia, medication side effects, and other possibilities. The most helpful treatment stems from understanding the problem first. Psychiatric evaluation should help discern this.
There could be but-- The first question I would have: what is thought to be 'causing' the flat affect? Low emotional reactivity is not a 'diagnosis, ' but is a sign or symptom of several sorts of difficulties. Flat affect is sometimes helped by medications, but that depends on the cause. Therapy might rarely be able to make a small impact on this. If needed, consult a health professional in person.
Meds and therapy. The most optimal combination for improving mental health issues is a combination of meds and psychotherapy. Flat affect, in other words no emotional expression can be presented in many cases, such as those with negative symptoms of schizophrenia as well as people who have difficulty expressing their emotions, as well as those who are repressing severe trauma.