Hey Shambha,
I noticed in your first post you mentioned that you noticed your symptoms after the summer was over and you went back to university. Do you think your symptoms started at that point, or did they start earlier and you just didn't notice them?
As far as your fatigue, I'm assuming you have had general blood work done, and everything is normal? One thing you may not have done is an endocrinological workup. There are some studies that state MDMA can acutely increase your cortisol levels (one study said cortisol increases up to 800% during MDMA), which may lead to pituitary burnout. Moreover, serotonin fibers are quite prevalent in the hypothalamus, and the hypothalamus controls the pituitary, so a serotonergic injury to the hypothalamus may also result in pituitary dysfunction. The can lead to a defiency in one or several hormones that may be affecting your energy levels, including cortisol, thyroid, and testosterone. I would check these if you have not done so, as they are pretty easy deficiencies to fix. Specifically you should check an AM cortisol level, TSH, T3, free T4, LH, FSH, and testosterone levels.
Are you taking pregabalin for insomnia? Its pretty atypical to use it for insomnia, and it may even be exacerbating your fatigue. Pregabalin is often used for neuropathic pain or fibromyalgia type conditions.
Also, decrease serotonin levels may be associated with increased sensitivity to pain and fatigue, so that you are no more fatigued than usual, but you feel it more.
As far as getting a PET scan, you would have to get a PET scan with a radiolabeled tracer that binds to SERT receptors, which is a test that is not widely available. Doctors are trained to not order tests if it will not change their management. If the PET scan demonstrated that you have decreased serotonin fiber density, what would you do differently with that information? Probably nothing more than you are doing now.
Take care Shambha. I hope things get better for you. No one deserves what you have gone through.