You refer to this problem as separation anxiety disorder but a more accurate diagnosis may be dependent personality disorder. In order to determine what disorder, if any, you have, it would be best to consult a mental health professional who could interview you in person and gather a detailed biopsychosocial history of your life.
As you have described this problem, it is significantly hindering your life. It seems as though you struggle to have the confidence to be an independent person. Fortunately, you recognize that a problem exists and are willing to seek help. The prognosis for such an open-minded individual who is receptive to treatment is excellent.
Many people your age struggle with developing their identities and gaining their independence. You might struggle with this issue moreso than the average individual but it is still a correctable problem.
You also mentioned that you had a reaction to taking a “synthetic” drug. You did not detail that situation any further but that experience may have had a negative impact on your psychological state of mind.
My recommendation is to receive counseling. A mental health professional could assist you in addressing this problem. It is important to seek help sooner rather than later. The main reason is because this problem is interfering with many aspects of your life and if left untreated would likely continue. The find help tab, at the top of this page, could help you to locate a mental health professional in your community. Please take care.