Hello and welcome to the forum:
Making the diagnosis of a "viral cardiomyopathy" can be difficult and patients are sometimes labeled as having had this due to the inability to provide a more exact explanation for the cause of the heart dysfunction. It is possible, as the NP told you though, that the MRI is "suggestive" ov myocarditis. However, you would need a biopsy to confirm this and this is often not performed because in most instances it wont change the way you are treated.
I am also intrigued by your frequent heart fluttering, afib, etc. This in its own right, if happening frequently enough, can cause heart dysfunction and should be aggressively treated.
Finally, you must get onto heart failure medications and have the doses increased to maximally tolerated levels. This will give your heart the best chance to recover.
Exercise is probably safe as your doc suggested and I agree with "listening to your body" and stopping when you begin to feel tired. Unclear if exercise can truly strengthen your heart but staying active is clearly beneficial for multiple reasons including in patients with heart dysfunction. You should also be sure to see a "heart failure specialist" at some point in addition to your general cardiologist.
I think there is a good chance at a good prognosis but you need to be in good hands.
All the best,
Dr. Rich
Also my cardiologist said I have no restrictions. I asked if I can exercise and he said yes to do what I can and stop when I feel tired. So would using an elliptical be okay? Can exercise help to strengthen my heart muscle?