Both the medicines are indicated for depression only. It is not clear from your communication if a diagnosis of depression was made in your case because your symptoms do not suggest so.
Mirtaz is a brand name that contains mirtazapine. It is used only in major depression. It is not a very safe drug with many side effects and hence should be taken only if
(a) a firm diagnosis of major depression has been made
(b) other conventional, safer medicines have failed.
Its side effects include: serious blood disorders (including the suppression of white cells), seizures, liver damage, pain in joints and muscles, retention of water in the body (oedema) leading to high blood pressure, fatigue and sedation.