Sorry, hunni, but most ADs have side effects to begin with. Each person reacts differently but you really do need to let the ADs get settled in your system. If you keep chopping and changing between meds, you're just going to feel a lot worse. It will also take 4-6 weeks for the mirtazapine to really start doing their job. The reason you feel drained at the moment is that mirtazapine is a sedative. Hopefully you've been told to take them at night? For most people, the drained feeling does lessen as your body gets used to the drug.
I know it's rough now but mirtazapine is a good drug. It's just that there's no quick fix for depression