When someone is diagnosed with a mental illness, there is always the possibility that they will get "better", especially with therapy, coping techniques, even medications and all the other ways that will alleviate the negative symptoms of the mental disorder. Yet, whether they will be "cured", depends on the mental illness. For example, there are some mental illnesses caused by genetics, deformations of the brain, and chemical imbalances: mental illnesses caused by these factors are never going to go away completely, but they can and will get better if an effort is put into it, even in most cases to a point where whoever in question can live a normal and happy life.
Being diagnosed means you are able to get the help you need. Chances are you still had your condition before being diagnosed. Though now that you are, professionals can help you can the correct medication and therapy. If you have been diagnosed, consider it the next step on your journey to achieving a better you.