The mechanisms of mirtazapine are different than any of the antidepressants you tried in the past.
Mirtazapine is an antagonist of most 5-HT receptors
and adrenergic receptors.
Unlike most conventional antidepressants, however, mirtazapine is not a reuptake inhibitor and has no appreciable affinity for the serotonin, norepinephrine, or dopamine transporters, nor is it an MAOI or have any efficacy at inhibiting/inducing any other enzyme for that matter.