While excessive use of X can lead to a temporary depression, what you are describing is a little bit different than serotonin depletion. If you were predisposed to depression, then excessive usage would have a more dramatic impact, of course. The fact is that your serotonin will replenish itself over time, and even in the most drastic cases just a few weeks is enough time, provided that the mechanism itself is not damaged. Now, high doses of X at once cause more damage than low doses over time. Case in point...I have used X for twelve years, but I do not use it excessively, nor do I use high doses. The years that I used high doses (200+mg/night) I had a more difficult time with short term memory.
The suggestion to seek medical advice has pros and cons. Right now there is a hysterical and unjustified attitude towards MDMA in general, and doctors tend to overstate the ACTUAL damage that is done by this drug. Some damgae is done, but it is significantly less than the damage done by average alcohol consumption.
Many here will begin constructing an argument against the last statement, but facts about this are readily available. May I suggest that you join the discussion boards at bluelight.nu and see what advice and insight is offered there. Often the worst place to get advice is from a doctor who has only READ about the drug. Remember, there is no substitute for experience.
The main problem you are exhibiting at present is simply low self esteem. You are punishing yourself for drug use, and excusing laziness by citing a fictional state of damage to your mind. I know about this, because I have tried that too.
Your mind can convince you of just about anything. Best way to counteract that is to TAKE CONSTRUCTIVE ACTIONS, regardless of how you FEEL.
It is what you DO that will overpower what you FEEL.
Doing nothing is the road to a quick death, both spiritually and physically.
For starters, read THE SPIRIT MOLECULE (about dmt), and then see what thoughts occupy your mind about this issue in general.