I cried every day for two months after my first boyfriend dumped me (and thought I was going to die...I didn't leave my room for a week and missed school, even though I had previously been a straight-A student). It was my senior year of high school and he was a year older and had already gone out of state to college and wanted to be free to check out his other options. I had been very depressed in the months leading up to the breakup and was suicidal for a short period. It was a very dark period in my life. I enrolled in counseling and took antidepressants for a year and learned how to be happy by myself again. I've had several serious relationships in the 10 years that followed (and some not-so-serious), and some of those breakups were very painful and some meant nothing to me (and it hasn't just been the breakups of serious relationships that has thrown me for a loop).
The good news is that nothing could really scar me after that first breakup; I knew I had made it through that, and I could make it through the next one, too. The fantastic news is that I found love again, and have had a chance to really evaluate myself and my relationships through the eyes of hard-earned life experience. I am now engaged to my best friend, a guy I have known since before I even started dating that first guy who broke my heart way back when, and it feels amazing to be so in love with someone with whom I share such a strong, long-term bond. We could never have had what we do now if we both had not been kicked around by life for awhile (and had each others' shoulders to cry on periodically in the process).
I second the other posters' advice to distract yourself and to indulge yourself in things that make you feel good. This is a really tough period. I have been through a lot, but I'm not sure anything that has happened to me thus far has been as painful as that first breakup. If you can, channel the negative energy you're feeling into something that really benefits you. I owe it to one of my breakups that I finally got my act together and made and started a 3-year plan to get into graduate school. (And it doesn't hurt that I expanded my social circle as a result.)
What I always tell myself for some comfort is: As long as the two of you are not separated by death, if it is really meant to be, at some point in the future you can always find each other again and start over. While I have often wished (and sometimes still wish) that those partners who have hurt me would suffer in some horrible way to reflect the pain I felt, I have also grown to appreciate the value of allowing both of us to move on, and later, to re-establish ourselves as friends. I know a few of my exes will give me the straight truth when I need it, because the dirty laundry aired between us means that nobody has anything to hide anymore. And some of them have just moved on without me, and I'm ok with that now that time has passed.
A breakup really is a period of bereavement; take heart, if you can, that you have the ability to feel so strongly and to participate in a moment so human, and do whatever it is that you need to do to get you into through the next five minutes until the pain subsides just enough that you can think about the five minutes after that. Take care of yourself, first and foremost, and it will get better. Your suffering is not in vain. Reach out to your support network (that includes us!) for support when you need it; we will do what we can to help you through a difficult time. If you are tempted to get back together with your ex, just try to clarify for yourself whether you're doing it because you are really a good fit for each other, or because you really don't want to be feeling the craploads of pain and suffering a real breakup entails. There have definitely been times for me when I preferred the bad relationship to the painful breakup, but those periods kept me from moving forward, too.
Finally, only if all else fails, I grudgingly admit to having resorted to the quick-and-dirty breakup remedy: The rebound. It's a terrible idea and it's almost guaranteed to be a mediocre relationship, but it's one of life's most potent distractions. And hey, by the time it's over, you're that much further away from the one that really hurt, so in that sense, it works. For use in desperate circumstances only. If you can muscle through it without a rebound, that is always my first choice.
Hang in there!!!