Hello~I am so sorry you are having such miserable symptoms. The only thing I can think of is you could have a hidden infection. Has the doctor run any tests such as blood work to see what you white blood count is? Possibly, allergies could be causing the problem as well, either food or environmental.
I wish I could be of more help, I hope you get the help you need soon. God bless you.
I had croup when I was 15. At the time I had undiagnosed autoimmune pernicious anaemia (vitamin B12 is essential for a healthy immune system).
"Teens who have not developed antibodies to the viruses that cause croup and those whose immune systems are compromised can develop viral croup." - Livestrong - Croup in teens.