Hi Ashlee!
If you have any doubts if a solo trip is fun, put them to rest now & go for it! Traveling the "world" solo can be awesome fun! You'll be in good company with many of your fellow guests that all share your love of Disney. Be sure to take full advantage of rides that offer Single Rider entrances like Test Track @ Epcot, Expedition Everest @ AK & Rock 'n' Roller Coaster @ DHS.
My #1 recommendation for solo travelers is to do some of the backstage tours & special experiences - click
here for a list. My favorite is the Backstage Magic Tour. You get to go behind the scenes of so many different places & learn some incredible insider info. Most people don't realize that many of the places are even on property.
Traveling solo is also a great time to try out a lot of restaurants. Many of them (even some of the finer ones) now offer counter service. Its a great way to meet people.
As far as Character Meals go - do it! Don't worry about feeling strange, the Cast Members at Disney are accustomed to solo travelers & will make you feel right at home. They'll be happy to assist you by taking pictures for you.
I'd also recommend Characters in Flight at DTD - a very cool experience!
Sounds like you will be celebrating turning 21 (a great age) so you'll be able to take advantage of some of the nightlife. There are a lot of places but be sure you check out Jellyrolls & Atlantic Dance @ BoardWalk; the lounge at The Wave @ the Contemporary Resort; Rose & Crown Pub @ the UK Pavilion at Epcot & the restaurants at DTD.
You may want to take the opportunity to pamper yourself at one of the incredible spas or just relax by the pool. Wishing you many magical memories :-)