We're sorry your muscles feel weak.
The word "diffuse" means "widespread" and refers to symptoms that are not localized to just one or a few areas. Instead, it is more or less all over, or at least in many areas. This guide is intended for people who feel weak all over.
This guide will deal with true weakness, meaning reduced power in the muscles. Symptoms of muscle weakness include difficulty rising from a chair, brushing your hair, lifting an object off a high shelf, or dropping things. It might cause drooping of an eyelid or difficulty smiling, depending on which muscles are weak.
People with other symptoms, such as fatigue (tiredness, feeling exhausted), sleepiness, or depression, will often say they feel weak and it can be hard to distinguish some of these from actual muscle weakness. So, if you aren't sure whether this guide is for you, start the guide and see if the symptoms it covers apply to you.
One more point before we move on: some people feel weak due to pain. Weakness and pain can certainly go together, but this guide is not focused on causes of diffuse pain.