Suffering from memory loss poss related to depression, anxiety (stemming from marijuana use) and disruptive sleep. remeron (mirtazapine) effective, is their better?

No: The best medication is the one that works for you. There are other medications that would have a similar effect. If Remeron (mirtazapine) is working well and you are not having adverse side effects, stick with it.

Insomnia might be due to several different reasons. First have a complete physical exam by a Dr. to rule out underlying biological problem. If no problem consult with a Clinical Psychologist for psychotherapy help & a Psychiatrist for medication, possible referral to a sleep study.