Re: lowered milk supply?
Throw out the formula, you don't need it! My babies were both extremely fussy and colicky in the evenings, and so are many other babies. Colicky evenings are very, very normal at 2 months. I actually think you are fortunate that your baby will nurse during that fussy time: neither of mine would and it made coping with the crying much harder. At least when there is a breast in the baby's mouth, she cannot scream at you!
Your age has nothing to do with the quality of your milk. Milk production is controlled by your pituitary gland, not your reproductive organs. Theoretically, a mom could lactate for as long as she lives, provided that she continued to nurse or pump. I have known moms who have successfully breastfed their babies in their forties and fifties (yes, fifties!).
Your instincts are absolutely correct that it is not a good idea to limit time at the breast. Some babies are fast feeders, some are slow feeders. Babies don't watch clocks, and neither should their mothers.
Finally, I also encourage you to see a doctor about your illness. A month of diarrhea might be more than a virus.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"