Short term memories are based on our working memory. Which are things we hold in our brain for a short amount a couple of minutest to facilitate a reward of some kind. Long term memory are that which we choose to keep from short term memory until we forget (retention loss) those memories to make room for more.
Modern universities are flawed in such a way that they tend to test long term memory with a level of detail that requires forgetting as annulment. Students memorize a set of facts for a specifically designed test without the intention of ever being able to recall those principles expect on the most general basis. Research and socratic methods of learning prompt internalization of facts and theories into part of the person's expert abilities.
Psychologist argue about the limits of long term memory. Of course the human brain is only so large so it can only encode a finite number of experiences some being forgotten to make room for more.
If you begin experienced an overly short attention span. Like forgetting what your supposed to be doing or becoming slightly fixated on a single task. That is to say your working memory doesn't seem to last as long as it once did and you find that which is intriguing now difficult to focus on you may be developing ADHD or any number of other conditions.
My mom slowly is forgetting me. Our interactions were limited to happenings years and years ago. Much of which she can no longer remember. Our monthly emails are not enough to maintain those diminishing memories. She has a mild form of dementia a cognitive impairment disorder worse than ADHD which may affect many different processes including long term memory. Her neurologist who also is a psychiatrist explained to her that it was common to forget the bad memories over time but the good memories are supposed to stick with us for all our lifetime.
A doctors diagnosis is required to determine the kind and state of the illness. For ADHD there are many somewhat safe medications like Adderall which can cause psychosis but is generally safe.