Relationships end. Sooner or later, people start to lose their interest / passion. It becomes a routine. It is VERY rare that two people can keep the flame alive for long times. Usually, it is said that there is a 3 year barrier, where the first crack happens (2.5 - 3.5 years kinda) where your stuff falls.
If you want my advice as a person with certain experiences, don't base your life on someone you love / loved. Learn your lesson from this one, base your life on thyself. Enjoy the moments you have with the one you love but when she/he etc is gone, life goes on. You should be the base multiplier, not your girlfriend.
Sounds selfish? Well, welcome to life.
Be crazy, don't get in any new relationship for at least a year. Have fun, get wasted, get over it. Don't expect to act normal or try to contain your depression, it will be there and depression has to be faced after break ups, bad events, traumas etc. Your fix is to let it spin off for a while until you feel the time is right to get back on track.
Remember, it is not only the good things, but all things that happen to us which helps us to shape our character and develop ourselves. This was a lesson, an experience for you.