Feeling down after a con, party, or major life event like graduating from HS is a very common reaction. I've experienced that myself many times. I think that things I've used to help include planning the next party or fun excursion, doing something distracting (depending on your personality it could be starting a project that will make you feel "productive", or watching some tv or a movie, talking to friends on the phone, making a diary entry, etc.) Taking a long walk or doing some other kind of aerobic activity is also very curative for the "blues". Also, try not to fall into the trap of blaming yourself or feeling guilty for feeling bad. Sometimes you will feel down. It's fine to try to do things to bring yourself out of it, but sometimes if you just kind of let it wash over you, it will go away on it's own. In any case, I hope you do feel more cheerful soon.