Welcome! It's great to see someone taking care of their credit at 21.
I'd get myself back on the Discover card. One of the factors in the FICO score is credit history. You just wiped out both (a) a 20-year credit history; and (b) a perfect track record on this card over the past couple of years. Having a credit history of only 3 months will really hurt your score. Of course, if anyone's paying attention they may notice that you were only 1 year old when you got the card,

although you could have been added at any time.
If you go back as an authorized user, make sure the balance never gets any higher. The utilization ratio (amount of credit used versus credit line) should be well below 30%. Currently the Discover is 26%. Let your parents know!
Also, it will help you if you have a small balance on your credit card rather than paying it immediately. You need to show that you can manage your credit, and if it looks like you never use your card, you aren't proving yourself. Keep up the perfect payment record!
Good luck, it's great to see someone starting on the right foot.