IT IS NOT IN YOUR HEAD. PERIOD.
many of us here were in the same situation with the same "diagnostic". I dont have much experience on how to get over this - i'm still learning but you should read something about the diet first. see some messages on adrenal fatigue and after you'll build step by step.
what helped me first hand was diet, lifestyle and rest. im still recovering. im 100% sure those more experienced will answer to you.
try calm yourself - dont mind about regular doctors - and learn as much as you can from here.
my question would be what happened lately since you've said 3 weeks ago was pretty much ok?