A very large dose of what? For example, if it's LSD you're talking about, most of it has speed in it making it harder to come down off of it. When you take a black market pharmaceutical product, you never know exactly what you're taking. So there's that. If it's Ecstasy, that can last a long time as well. In the 60's when people first started using some of these pharmaceuticals to get high, the drugs were usually at least pure, but that lasted about a week. Once the profit motive set in, they cut the drugs very often with cheaper things that intensified the high to make more money. Now, you probably don't have brain damage, as someone mentioned, from one experience -- that would be very very unlikely. Most likely you're suffering form the trauma of the experience because you didn't have anyone with you to talk you down from it or you were prone to having a bad experience, which many of us are but don't find out until we have one. You've now possibly conditioned yourself to expecting to feel some of what you felt after this very traumatic experience, and a therapist might be able to help you untrain yourself back to where you were. I'm guessing you can and will do this. The only people I knew back in the day who suffered permanent problems took a lot of drugs or took a lot of different drugs at one time. What you have learned is you're probably not one who likes being really high and so this just isn't your thing, which is fine. Even pot turns on people, as this forum's archives will attest to.
You would have to see a professional to determine if you even had any brain damage.
Lots of depressions and anxiety (which go hand in hand feeding off each other) come from a new way of looking at something, so your issue may just be temporary if you can find a new way to approach the subject. A therapist might be a good idea to see if they can help you, since you have lost a lot of time feeling bad just hoping it will go away but it hasn't so far.