Paxil-Remeron combo (Paroxetine-Mirtazapine)

Hey there, I'm May.
I'm 17 and a half and have been on medication for MDD and anxiety disorders (GAD and PTSD) for over a year.
I'm currently on 40mgs of Paroxetine (morning) and 30mgs of Mirtazapine (night). I've been on the former since October and the latter for a bit over two months, each doses have been upped twice.
The issue is, I usually feel better up to three weeks after changing my doses, but then I fall back into depression. I've had three very tough, long and severe depressive episodes these past 5 years, this one being my third one. Others have been shorter and less severe.
I'm wondering, for those taking this combination of drugs, how's it working for you? Have you experienced long-term effects or a similar lapse after a while?
I'm getting desperate given how debilitating my depression and anxiety symptoms are...
PS: I'm also on .25mgs of Xanax daily.

Wishing you all well,
MayBug.