Hi there... I'm sorry to hear about your troubles, it sounds like you are concerned and frustrated. First off, if you have noticed a change in your functioning, then something is probably going on, although it doesn't necessarily mean that it's a mental illness. Have you talked to your primary care physician about what you're going through? A discussion with a medical professional combined with a medical work-up (blood tests, etc) is a good first step to rule out any medical problems that could be causing your problems.
AHDH is an attention deficit disorder
that causes problems with attention/ concentration, distractibility, etc. Memory problems aren't a common symptom, although people with ADHD can have trouble retaining information because they have trouble focusing (e.g., they have trouble reading because they are distracted/ can't focus on what they are reading so they have trouble retaining the information they've read). ADHD is usually diagnosed in childhood, so these problems that you're having should've been present since then. Did
you have any of these symptoms when you were in school?
Bipolar Disorder, which you also mentioned, is a mood disorder characterized by periods of elevated mood (e.g., increased energy levels, decreased need for sleep, racing thoughts, rapid/ pressured speech, feelings of euphoria or irritability) and periods of depressed mood (basically the opposite of the symptoms described above). From your description, it sounds like you may have experienced some of these symptoms -- what do you think?
An MRI wouldn't be able to diagnose a mental illness, like ADHD or Bipolar Disorder, and I think a doctor would recommend a full medical work-up before referring you for an MRI. So again, if you haven't already done so, I would recommend making an appointment with your primary care physician as a first step. If you have seen your physician recently and the results were all normal, then I would recommend making an appointment with a mental health professional -- like a psychologist or psychiatrist for a full psychiatric evaluation. I am only able to provide you with general information to help answer some of your questions, but I can't provide any diagnoses without doing an evaluation in person.
I hope this information has been helpful. If you have further questions, I'm happy to answer them.