A few questions to consider.
Have you taken anything to alleviate the cold/flu symptoms that might interfere with your meds?
Have you missed/ forgotten to take your meds at all, or have you had any vomiting?
Just asking because if you're not absorbing your meds efficiently because of your illness then it could explain things.

There is a link between viral illness and depression, it isn't uncommon for people who have never suffered MI to become clinically depressed following a viral illness. Glandular Fever is often associated with post viral syndrome. Have you seen your doc? 3 weeks is quite a long time for a virus to be bugging you (pun intended), if you've not had Glandular Fever get your doc to run a blood test.

In the meantime, take it easy, plenty of fluids and hope you feel better soon.