The outcome with glandular fever is much more promising than with other causes and many of the people who fully recover have had that. Also you are still young and that is good too. Getting adequate rest and listening to your body, as above, are really important, but the most important thing is to be careful when you start to feel better! That is the point where people (myself to my regret though I did not know any better then) who are desperate to do things - live a life - overdo it. Go canny and you can still have a decent life as ME can be managed even if it does not go altogether.
Just remember that being careful and aware that there is no rush to life, you have many years left to do all the things you want even if you have to spend a few years taking things easy.
Also, look up the work on CPET testing, the Workwell Institute is a good place to start, and use a heart rate monitor. Our aerobic respiration system is broken so we have to use the anaerobic system if we do too much. It is the byproducts of that that lead to damage.
And keep in mind that it is not your fault you are ill and avoid spending a fortune on "miracle" cures that well meaning friends and family encourage. There are a lot of people out there just ready to sell quackery to us.
Oh and in case you don't know. STAY AWAY FROM GRADED EXERCISE. That is the thing most likely to make you get worse.