What a great question! I was a psych major, so everything to do with the brain is interesting to me.
Your memory problems of things done during the period when you were drinking as a teen would certainly have interfered at the time with your brain's ability to store memories. Any chemicals in your system will interfere with your ability to pay attention, sleep, and then store the memories during the sleep cycle where that happens.
It sounds like you have memory issues at times other than that as well, though, so it could be that the drinking at such a young age damaged something that is involved in your brain's ability to store memories overall. Depends on how much you drank, how often, and your individual genetic profile and family history.
It is also possible there is something else, unrelated to the drinking, that is going on. I'm sooooo glad that you said you are planning on seeing a doctor. I am not able to give any kind of guess on this kind of medical condition, other than to say that any kind of chemical imbalance in the body affects the brain and memory. In addition, anything like depression, anxiety, etc., will specifically affect your brain's ability to store memories, and sleep disorders, etc., will affect your brain's ability to get to the later stages of sleep and store memories.
I am also dismayed at my inability to remember things. I haven't actually TRIED, but I'm a writer, and I would love to write a novel based on my life experiences, or autobiography, and I'm not sure I could remember enough to do it. I'd have to reinvent a lot of it based on my memories of the big picture. Maybe we're not supposed to remember details? Maybe memory is a primitive survival mechanism that is meant only to keep us from sticking our hands in fire, jump into murky water, etc., and we expect too much of it?
A journal is wonderful. Hand it down to your great great great grand children!!! I wish I had one of my ancestors!
** peace **