I want to express the idea that I'm really bad at recollecting something from my memory, but "I have a bad memory" sounds a lot like I remember (maybe fairly well) something bad that happened to me.
A common idiomatic expression is having the memory of a goldfish.
To have an exceptionally poor memory. An allusion to the myth that goldfish can't remember anything for longer than a few seconds.
Mom has the memory of a goldfish, so don't bother asking her—there's no way she'll remember.
the memory of a goldfish
(colloquial) A very poor memory.
Another example from an article on Huffpost:
I went from loving backpacking and sailing to struggling to make it to a 50-minute lecture; from being able to recall nearly any dialogue I had heard to having the short-term memory of a goldfish.
Note: It is scientifically proven that goldfish have a fairly good memory:
In reality, goldfish (Carassius auratus) have much longer memories — spanning weeks, months and even years, Brown said. - livescience.com
The idiom is based on a "myth" which is even in the definition. Scientific inaccuracy doesn't change the colloquial/idiomatic usage. Blind as a bat is another popular idiom like this, as bats are actually not blind and some species even have particularly sharp vision that they use it together with their echolocation.
Of course you can say:
- I am bad at remembering things. ;-(
But more commonly, you use:
- A student with a poor memory may struggle in school.
And if you have a really bad memory:
a memory/mind like a sieve
- I have a memory like a sieve.
The word memory has the double meaning of:
someone’s ability to remember things, places, experiences etc
something that you remember from the past about a person, place, or experience
I have a bad memory. = I am bad at remembering things. (= a poor ability to remember things in general)
I have bad/terrible memories of my stay in India. ( I remember bad things...)
As you can see, there are lots of options, but "I have a bad memory" is also just fine. It does have that ambiguity, as a matter of the meanings of the words, but people will know what you mean. One reason for this is that you would be a rare person indeed if you did not have multiple painful memories. Saying "I have a bad memory" in that sense is kind of like saying "I have a shirt." But saying "I have a bad memory" in the sense of having poor recall actually is informative, and so people will interpret it that way.
A good word for this is forgetful. DialFrost’s suggestion of absentminded or absent-minded is another great one. We often describe intelligent people as absent-minded. A few of the others have, to me, slightly too negative a connotation, so I would be careful with them. You might not be portraying yourself in the best light.
You could also say, you have a poor memory rather than a bad memory, which doesn’t have the same ambiguity.
"I have a bad memory" sounds a lot like I remember something bad that happened to me.
While that is grammatically speaking a correct possible interpretation, a listener will be perfectly able to understand you given the context of the conversation you're having.
Generally speaking, people are not going to state that they have had a negative past experience without either (a) the conversation clearly leading to this topic and/or (b) you not elaborating on that specific experience. If you just state that "you have a bad memory", it is clear that you mean that you don't remember things well.
If you'd still rather avoid the ambiguity anyway, you could opt for alternatives such as "my memory is bad", which more clearly refers to your memory ability (rather than a particular memory) being bad.
Alternatively, you could change the adjective to a less ambiguous one. Poor, flawed, short, ... are more clearly descriptive of your ability to remember rather than a particular memory you have.
You have choices:
- I have a poor memory.
- I am forgetful.
- There are holes in my memory.
- My memory is often a blank.
Those express the idea that memories are missing. Sometimes you want to say that you remember things that didn't happen just the way you remember them:
- My memory is faulty.
- I misremember things.